1911 Encyclopædia Britannica/North America
NORTH AMERICA. In the article America a brief geographical survey is taken of the two continents which bear this name; and their points of similarity and contrast are broadlyComparison of North America and Eurasia indicated. When North America is compared with the northern continents of the Old World, an important correspondence is found between it and the greater part of Eurasia; but here the corresponding parts are reversed, right and left, like the two hands. The Laurentian highlands agree with Scandinavia and Finland, both having escaped deformation since very ancient times. A series of water bodies (the Great lakes in North America; the southern Baltic, with Onega, Ladoga, &c. in Europe) occupy depressions that are associated with the boundary between the very ancient lands and their less ancient covering strata. The old worn-down and re-elevated Appalachian mountains of south-eastern North America agree well with the Hercynian mountains of similar history in middle Europe (Ardennes, Slate mountains of the middle Rhine, &c.), each range entering the sea at its Atlantic end (in Nova Scotia and Newfoundland; in Brittany, Wales and Ireland), and dipping under younger formations at its inland end. Certain younger ranges—seldom recognized as mountains because they are mostly submerged in the American mediterraneans (Gulf of Mexico and Caribbean Sea), but of great absolute relief and with crests rising in the larger West Indian islands may be compared with the younger ranges of southern Europe (Pyrenees, Alps, Caucasus) bordering the classic Mediterranean and the seas farther east. The central plains of North America correspond well with the plains of Russia and western Siberia; both stretch from great enclosed water bodies on the south to the Arctic Ocean, and both are built of undisturbed Palaeozoic strata toward the axis of symmetry and of younger strata away from it. Finally, the Western highlands of North America may be compared with the great mountain complex of central and eastern Asia. In this remarkable succession of resemblances we find one of the best proofs of the continental unity of Eurasia. Moreover, the resemblances thus described controvert the idea, prevalent when geology was less advanced than to-day, that the New World of civilized discovery is an “old world” geologically, and that the Old World of history is geologically “new.” Both worlds are so old, and both share so well the effects of successive geological changes from the most ancient to the most modern periods, that neither can regard the other as older or younger than itself.
There are several climatic similarities between North America and Eurasia. The Appalachians and the Hercynian mountains of middle Europe both contain extensive coal deposits of similar geological age, thus indicating a climatic and geographic resemblance at a time of great antiquity. The Laurentian highlands and the Scandinavian highlands were both heavily and repeatedly glaciated in recent geological times, and the ice sheets that crept out on all sides from those centres spread far over the lower lands to the south and away from the axis of symmetry towards the continental interior, scouring the highlands and leaving them rocky and barren, strewing extensive drift deposits over the peripheral areas, and thus significantly modifying their form and drainage; while the much loftier mountain ranges of western America and central Asia suffered, singularly enough, a far less extensive glaciation. At the present time, the plentiful and well-distributed rainfall of the continental border on either side of the Atlantic is succeeded by an increasing aridity towards the continental interior, until the broad plains that rise towards the distant mountain complexes are comparatively barren or even desert. Within each greater mountain area extensive interior drainage basins are found holding salt lakes, and the recently determined former extension of these lakes in Central Asia agrees with the well-proved extension of Pleistocene lacustrine conditions in western North America.
The following sketch of the geological development of North America considers the larger physiographic divisions already indicated.
The extensive area of ancient crystalline rocks (Archean), stretching from Labrador past Hudson Bay to the Arctic Ocean, is of greatly disordered structure, and hence must have once had a mountainous form. Moreover, the crystalline texture and deformed foliation of the rocks prove that the surface now seen was once buried Laurentian highlands.deep beneath the surface of an earlier time, for only at great depths can such texture and foliation be acquired. Both these lines of evidence lead to the conclusion that the moderate relief prevalent over the existing Laurentian region is the work of persevering erosion during a long continuance of dry land conditions, and hence that the region must be regarded as a worn-down mountain system. The worn-down old land is gently overlapped, chiefly around the south and west, and south of Hudson Bay, by very early Palaeozoic strata which rest upon the eroded surface of the crystallines, thus proving that the destruction of the ancient mountains had already been accomplished before some of the oldest fossiliferous formations of the world had been deposited. All the evidence goes to prove that from then to now the Laurentian region has been relatively quiescent. In all subsequent time there have been here only moderate oscillations of level, one of which allowed the transgression of the ancient sea in which the overlapping strata were deposited, while another of much more modern date gave the region its present highland altitude (1000 to 2000 ft.; mountains near the Labrador coast, 8000 ft.), again offering it to the forces of erosion.
It is this ancient Laurentian area that the earlier geologists named the “Continental Nucleus," as if it had been the first part of North America to rise from the primeval waters of an assumed universal ocean. The “Archean V,” formed by the two arms of the Laurentian oldland stretching from Labrador to the Arctic, between which Hudson Bay is included, has been repeatedly described as the oldest area of the continent, the beginning around which many later additions have built the existing outlines; and as such it has been adduced in favour of the theory of the permanence of continents. But when thus stated, the half of the story in favour of this theory is not told. Hudson Bay is not due to a primitive failure of elevation between the arms of the “Archean V”; it is not a deep basin whose floor has never emerged from the primeval ocean, but an ancient and comparatively shallow depression in a pre-existent land, over which the sea flowed as the surface sank below sea-level. South and west from the “Archean Nucleus,” the Cambrian strata of the medial plains of North America are found to lie, wherever their base is discovered, on a foundation that possesses all the essential features of the Laurentian oldland. This relation is found all around the Adirondack mountains in New York, along the Appalachians southward to Georgia, through the Mississippi basin in Wisconsin and Missouri, and beyond in Texas, and farther west in the Black Hills, as well as certain points in the Rocky Mountains region. Hence the pre-Cambrian land surface of the continent must have had not only a vastly greater area than was formerly attributed to it, but also an earlier origin; for at the time when it was thought by the older geologists to be first rising from the primeval ocean, it is now proved to have been slowly sinking after a prolonged land existence. The crystalline Archaean rocks in the Laurentian region and its scattered fellows cannot possibly be explained as a primitive sea bottom, rising above sea-level to make the beginning of a continent and receiving Cambrian strata upon its still submerged borders, but only as portions of an already old and deeply-denuded land area, which was in pre-Cambrian time much larger than the visible Laurentian area of to-day, and which was reduced to perhaps half its primeval dimensions by a gradual submergence beneath the transgressing sea in which the Cambrian sediments were laid down. We are thus led to believe that much of the continent of to-day was a continent in the earliest geological times, and that the seas which partly covered it in Palaeozoic and Mesozoic time were due to partial submergence, not to partial emergence. Furthermore, all the marine strata that now stretch over a large part of what is believed to have been the ancient continental surface are of relatively shallow water origin; none of them bears any close resemblance to the deposits of the deep oceans that have been so well studied of late years. Hence the Palaeozoic and Mesozoic seas of North America were not deep oceans, and as far as this continent is concerned it is by no means admissible to assume, as some of the earlier geologists did, that the position of continents and oceans have repeatedly changed places. The testimony of the rocks is decidedly in favour of Dana's view that continental masses are relatively permanent.
The early history of the Laurentian region has been dwelt upon because of its great importance in the history of the continent, and because its history has so generally been misunderstood. To these reasons may be added a third: through Palaeozoic and Mesozoic time the history of the Laurentian region is for the most part a blank. Records are wanting from the early Palaeozoic to the Pleistocene, when the Laurentian uplands became the centres from which the ice sheets of the Glacial period spread out on all sides. As a result of this late chapter in the history of the region, the weathered soils of earlier periods were swept away along with an unknown amount of firm rock, leaving bare ledges, scattered boulders and gravelly drift to-day upon a rugged upland without mountains (except in north-east Labrador), but diversified by innumerable knobs and hollows. The drainage of the region has thus been thrown into disorder; large and small lakes and marshy hollows abound; the streams are repeatedly interrupted by rapids, and frequently split into two or more channels, enclosing islands many miles in length. They are the only highways of this thinly inhabited region.
The Appalachian province is a generally hilly and mountainous belt, stretch in from Newfoundland to Alabama. It seems for the most part to have belonged in the earliest times to theAppalachian highlands. great pre-Cambrian land area, of which the Laurentian highland is the more manifest representative; for where-ever the basal members of the Palaeozoic sedimentary series are found in the Appalachians, they rest upon a floor of denuded Archean rocks, and the lowest layers are largely composed of Archaean detritus. This province must, however, be set aside from the undisturbed Laurentian region because of the repeated movements of depression, deformation and elevation that it has suffered, generally along a north-east south-west trend, causing the successive alternations of heavy deposition, and almost equally heavy denudation that have prevailed with varying intensity during the whole stretch of geological time covered by the fossiliferous record. The earliest important mountain-making disturbances interrupted the conditions of deposition in Cambrian time, and produced what has been called the Green Mountain system. A later, and probably greater, disturbance, with its climax at the close of Carboniferous time, established the Appalachian Mountain system; but, as understood to-day, the “Appalachian revolution" of the older geologists should be regarded as a long-lasting process, perhaps intermittently enduring as long as the whole of Carboniferous time. A subordinate period of deposition and deformation occurred earl in Mesozoic time, marked by the accumulation and disturbance of several basins of the Newark formation, roughly corresponding to the Triassic of Europe.
The Appalachian mountains of to-day were formerly regarded as the unconsumed remnants of the chief Appalachian uplift; but it is now generally agreed that Mesozoic erosion reduced the greater part of the range to a lowland of moderate or small relief, leaving only isolated groups of subdued mountains in the areas of the most resistant rocks, and that the altitude and form of the mountains of to-day are chiefly the result of the Tertiary elevation and dissection of the previously worn-down mass—the additional height thus given in Tertiary time to the pre-existent subdued mountain groups making them now the loftiest areas of the range, as in the White Mountains of New Hampshire (Mount Washington, 6293 ft.) and the Black Mountains of North Carolina (Mount Mitchell, 6711 ft.). It is interesting to note that the axis of Tertiary elevation is nearly parallel to and closely associated with the axes of the earlier disturbances, but it lies somewhat to the north-west of its predecessors, and therefore involves considerable areas of flat-lying Palaeozoic strata on the inner side of the previously disturbed belt from New York to Alabama, thus producing what is known as the Alleghany plateau (altitudes, 2000 to 4000 ft.). It should be added that the Ozark plateau of Missouri and the Ouachita mountains on the south, in Arkansas and farther west, are related to one another in much the same way as the Alleghany plateau and the middle ranges of the Appalachians—the two pairs corresponding to a remarkable degree in regard to conditions of ancient accumulation, medieval deformation and denudation, and more modern uplift and dissection; it is, therefore, admissible to classify this western group of uplifts as an annex to the normal Appalachians. Numerous and extensive coal seams occur in the worn-down Appalachians of Nova Scotia, Pennsylvania and Alabama, as well as in the Alleghany plateau from Pennsylvania to Alabama, and in the extension of the same strata through the Ohio and middle Mississippi basins.
The eastern coast of the continent has a rocky and ragged shore line from Maine to Greenland, with numerous submerged lowlands and valleys forming bays, and as many uplands and ridges outstretching in promontories and islands; this being the result of the summation of many movements of the land, whose total gives an increasing measure of depression to the north, where an archipelago at last replaces what was probably once a corner of the continent; but the measure of the depression is uncertain, because of the doubt regarding the depth beneath sea-level to which the Pleistocene glaciers may have worn the pre-Glacial valleys. South of New England, along the Atlantic coast, and around the border of the gulf into Mexico, the dominating movement of the land in late geological periods has been upward with respect to sea-level, whereby a former sea bottom, on which the land waste of Cretaceous and Tertiary times had been outspread, was revealed as a coastal plain, across which the rivers of the former land area now extend their courses, from the old shore line to the new. Part of the same plain, still submerged, forms the “continental shelf” of the mid-Atlantic border. Florida seems to be a projecting swell of this shelf, around whose extremity coral reefs have been added, but whose greater mass is still under a shallow sea cover. Along the ragged coast in the north a moderate and very modern movement of elevation has laid bare clay-floored lowlands that were lately beneath the sea, as in the plain of the lower St Lawrence valley, while along the coastal plain of the south a slight movement of depression has drowned a number of low valley floors, producing shallow arms of the sea, as Chesapeake Bay, Albemarle and Pamlico Sound and Mobile Bay. All the coast south of New York is low, and a great part of it is fringed with wave-built sand-reefs.
The great complex of mountains in the Western highlands sometimes styled the Cordilleras of North America (the Rocky Mountains being the eastern members of the system in the The Cordilleras of North America. United States and Canada), differ from the Laurentian and Appalachian regions in having suffered numerous disorderly movements at dates so recent that the existing relief of the region bears a significant relation to its irregular uplifts; a relation that doubtless once obtained in the older mountain areas of the east, where it has now been obliterated by erosion. It is not, however, only in modern geological periods that mountain-making disturbances have prevailed in the regions of the Western highlands; their geological history is one of repeated and long-continued movement—the ruins of the more ancient upheavals supplying materials for the strata of newer ranges. For example, in Canada an axial belt of ancient rocks is bordered on the east and west by stratified formations of enormous thickness (40,000 to 60,000 ft.), those on the west including a large share of contemporaneous volcanic materials; all three belts having been deformed and upheaved, as well as deeply dissected in the later chapters of geological time. It is, however, important to note that the interval between Palaeozoic and Mesozoic time, in which mountain-making disturbances were so general in western Europe and eastern North America that the older geologists thought them to be of world-wide extent, was here generally passed over in relative quiet, so that continuous sedimentation produced in certain districts a conformable series of deposits from Silurian to Cretaceous time. Furthermore, the Carboniferous period, which gained its name from the extensive coal deposits that were then formed in western Europe and eastern North America, was a marine limestone-making period in the Cordilleran region.
There is here exemplified, as might be expected in a region extending over 3000 m. from Alaska to southern Mexico, and measuring over 1000 m. in breadth at its middle, a great variety of plateau and mountain structures. The broad upheaval of adjacent blocks of earth-crust without significant tilting or disturbance has produced the plateaus of Arizona and Utah. Some of the simplest and youngest mountain ridges in the world are to be found in the broken and tilted lava blocks of southern Oregon. Tilted blocks on a larger scale, much more affected by processes of sculpture, are found in the lofty St Elias Alps of Alaska, the site of some of the greatest glaciers in the world. The wall of a huge fracture, now elaborately carved, constitutes the western slope of the Wahsatch range, facing the desert basin of Utah. Ranges of a relatively simple arch structure are seen in the Uinta mountains of Wyoming and Utah. Arched upheavals also characterize the front range of the Rocky Mountains proper in Colorado and Wyoming and in the Black Hills of South Dakota, bending up the strata of the adjacent plains in the simplest fashion, and producing dome-like mountains, now deeply dissected by outflowing consequent streams. A remarkable change in the structure of the Rocky Mountains occurs north of the Missouri river in Montana and northward into Canada, where the front range is of synclinal or trough structure, with the youngest instead of the oldest rocks along the axis, while the strata of the plains are bent down and overridden in the most abnormal manner. Indeed, mountain structure occurs of so great diversity in various parts of the Cordilleran region as to elude general description. The disturbances extend directly to the western coast line, including not only the coast range of California, but the peninsular area of Lower California (belonging to Mexico) and the detached mountainous islands of British Columbia and Alaska.
Volcanoes of commanding form here and there dominate the plateaus and mountains. Orizaba, Popocatepetl and their neighbours, terminating the Cordilleran system in Mexico; Mount San Francisco, bearing snow and Arctic plants above the nearly desert plateau of Arizona; Mount Shasta, with small glaciers in northern California; Mount Rainier, with extensive glaciers surmounting the Cascade range of Washington; Mount Wrangell in Alaska, and farther on the many cones in the curved chain of the Aleutian islands: all these have been heaped up around vents through which their lavas rose from some deep source. Vast lava Hoods have been poured out at different times. The southern part of the Mexican plateau is built up in large measure of lava sheets, capped with volcanoes. Extensive lava beds, barren and rugged, cover large areas in north-eastern California. The basins of Snake and Columbia rivers in Idaho and Washington are flooded with older and more extensive lava sheets, whose borders lap around promontories and islands of the “mainland.” Still older lava-flows in British Columbia are now deeply dissected by the branches of Frazer river, and remain only in disconnected upland areas. High plateaus in Utah are protected by a heavy lava capping, the result of great eruptions before the plateaus were uplifted. Here and there rise dome-like mountains, the result of the underground intrusion of lavas in cistern-like spaces, forming “laccoliths,” and blistering up the overlying strata. Thus, by mountain upheaval or volcanic eruption, great altitudes have been gained. Where the uplift has been strong, ranges of truly Alpine form with extensive snow-fields and glaciers occur, as in the Selkirk range of Canada (now traversed by the Canadian Pacific railway), and again in Alaska. Heights of 12,000 and 14,000 ft. are exceeded by numerous summits in the central part of the system; but the dominating peaks are found far in the north-west and in the south. Several mountains in Alaska exceed 18,000 ft. (Mount McKinley, 20,300 ft.; Mount Logan, 19,540 ft.; Mount St Elias, 18,000 ft.); and the great Mexican volcanoes rise nearly as high (Orizaba, 18,250 ft.). Widespread plateaus maintain upland altitudes of more than a mile over vast areas.
As in all regions of great altitude, the erosion of valleys has progressed on a magnificent scale in the Cordilleran region, and the actual form of many of its parts is more the result of sculpturing than of uplifting. The plateaus of Arizona are traversed by the deep cañons of the Colorado river and its branches, at places 1 m. deep, and with elaborately carved walls. Upon the plateaus themselves, long and ragged cliffs of recession attest an even greater work of erosion than the cañons. In all the mountain ranges except those of youngest uplift, valleys have been actively eroded, sometimes producing steep peaks as in Mount Assiniboine (11,500 ft.) in the Canadian Rockies, rivalling the Swiss Matterhorn in sharpness of form; but the greater number of summits have been worn to roughly pyramidal form between wide-flaring valleys, and the mountain flanks have thus come to be extensively covered with rock waste lying on slopes of relatively uniform declivity. Some of the ranges are in a second cycle of dissection, having been once worn down to moderate relief and now being elevated for renewed erosion; the Sierra Nevada of California is believed to be, in part, of this history, having at least in its central and northern parts been well reduced and now again enjoying a mountainous character in virtue of a later slanting uplift en bloc, with rapid descent on its eastern fractured face. Other ranges, almost completely worn down, still remain low, as in south-eastern California, where they are now represented by gently sloping rock floors veneered with gravel and retaining only small remnants of their original mass still unconsumed; thus the end, as well as the beginning, of the cycle of erosion, together with many complications of its progress, are illustrated in different parts of this great and varied mountain system. In the fjorded coast of Alaska, as well as in the higher northern ranges, signs of intense glacial erosion are seen in the cirques at the valley heads and in the discordant junction of the “hanging” lateral valleys and the deep trunk valleys—the floors of the former being cut off on the walls of the latter.
Fitting complements of the deeply-eroded mountains are found in the great accumulations of mountain waste now occupying basins of depression between the various ranges, as in Mexico, Utah, Nevada, Montana and elsewhere. Erosion and transportation here combine to build up the floors of the basins with the waste of the surrounding highlands; a result that is peculiarly beneficial in Mexico where the climate of the plateau basins is rendered relatively temperate by reason of its altitude, and where the surface is easily habitable by reason of its small relief. In the larger depressions, as along the boundary of the United States and Mexico, isolated ranges frequently rise like islands over the plain of waste that has been built up on their flanks. Shallow saline lakes or playas (wet-weather lakes) without outlets lie on the lowest parts of the waste-filled basins; their failure to overflow in rivers discharging to the sea being less the result of enclosure by barriers than of deficiency of rainfall; for it is chiefly in the arid region that the waste-floored basins are best developed. Indeed, the rainfall is often so scanty that the streams from the mountains—where most of the little precipitation occurs—often fail even to form lakes, withering away on the waste plains. In all these cases, the wash of rock waste from the mountains remains on the continent and builds up the basin plains, instead of being carried away from the land to form stratified sediments on the sea floor. The habit of gathering mountain waste in interior basins that characterizes so much of the Cordilleran region to-day is only the continuation of an earlier practice, for extensive basin deposits of Tertiary date are found in many parts of the Cordilleran region; some of them are famous for preserving vertebrate fossils, such as those of the many-toed ancestors of the horse.
Between the loftier western highlands and the lower eastern highlands (Laurentian and Appalachian) lies a great extension of medial plains, stretching in moderate altitude from theThe Medial Plains. Arctic Ocean to the Gulf of Mexico, and having in their middle a breadth of 1500 m. They are composed throughout of nearly horizontal strata and mark a region long exempt from strong disturbance. Although for the most part floored by marine formations, their structure and composition indicate, as has already been said, relatively shallow water. The ancient sea that once occupied the middle belt of the continent therefore had little likeness to the abysmal oceans, but resembled rather the shallow ocean margins that to-day overlap various continental masses—the largest example of this kind now existing being between Asia and Australia. The eastern part of the plains is under laid by Palaeozoic strata, already mentioned as having been laid down upon the subsiding Archaean continent or folded in the making of the Appalachians; coal beds are here included in the Ohio and middle Mississippi basins. The area of the western plains remained submerged to a later date, preserving a stretch of marine waters to the end of Mesozoic time, and thus resembling the lowland belt of western Asia, which was similarly covered by a broad and a shallow arm of the ocean extending from the Arctic to the European mediterraneans until a late geological date. The surface of the medial plains is not always so even as might be inferred from their name. Both the eastern and the western areas have been extensively denuded, even to the point of being reduced to peneplains. Their present altitude is not so much the result of their original uplift from the sea as of a later elevator movement. The great river basins, for which North America is famous, have thus been formed between the eastern and western highlands—the Mississippi receiving the drainage of a vast area (about 1,240,000 sq. m.) for discharge to the south, while the Saskatchewan and Mackenzie gather their waters from somewhat less extensive areas in the north. Pleistocene glaciation covered the plains of the Ohio, upper Mississippi and Winnipeg districts with extensive deposits of ice-laid or water-laid drift, furnishing a generally smooth surface and a fertile soil: here are the true prairies—treeless, but richly grassed.
The traditional continuity of the Cordilleras of North and South America has been broken by investigations in the isthmian portion Central America and the West Indies. of the northern continent. The structural peculiarities of the western highlands of North America may be traced only to the east and west belt of great volcanoes by which the plateau of central Mexico is terminated on the south. The ranges of the Andes fail to reach Panama, from which the nearest one is separated by the valley of the Atrato. The two Cordilleras are out of line with each other, and their ends are some 1200 m. apart. Central America, the West Indies and various submarine ridges by which the islands are connected with one another and with the mainland to the west, as well as certain ranges along the northern margin of South America, all belong together in what has been termed the Antillean mountain system, in which east and west trends of late geological date predominate, with abundant volcanic additions on the Pacific border of Central America, and along the eastern end of the system in the Windward islands of the Lesser Antilles. The unity of this system has been until recently overlooked partly because the Antillean ranges are for the most part still under water, and yet further because the volcanoes which form the strongest reliefs of the isthmian region are so arranged along the Pacific coast as to suggest the continuity of the Cordilleran systems on the north and south; but these volcanoes are really only superadded to a foundation of quite another kind. Geological studies on the mainland and on the islands have shown that both fundamental structure and surface form are not Cordilleran; and numerous soundings in the adjacent mediterraneans suggest that the islands are best interpreted as the somewhat denuded crests of great crustal ridges. The warm waters that bathe the West Indies come with a high temperature from the equatorial Atlantic, and favour the growth of corals along the shores. Fringing and elevated reefs are known on many of the islands. The Bahamas are the slightly over topping parts of a broad platform of coral and other calcareous marine deposits, of which the greater area constitutes extensive shallow banks, which descend b a steep slope on the north-east to great depths in the Atlantic. The lowlands of Yucatan resemble Florida in being the emerged part of a much larger mass, of which an equal portion is still under water in the shelf around the Gulf of Mexico. All this region is luxuriantly productive and is advantageously surrounded by waters which would be barren and desert, like the Sahara, if replaced by lowlands. The active volcanoes on the Pacific slope have built many cones and uplands, some of their historic eruptions having been of terrible violence. Thus Lake Nicaragua, once a bay of the Pacific, has been cut off by volcanic deposits, leaving only the Gulf of Fonseca open to the western ocean, raising the level of the lake behind the barrier and turning its discharge eastward to the Caribbean Sea across what was once the inter-oceanic watershed.
The successive crustal movements by which the land area of what we now know as North America has been increased and connected Rivers.have determined the growth of several great river systems through which the broader part of the continent is drained. The movements that resulted in the emergence of the Plains had the effect of engrafting many ancient rivers of moderate size upon trunks of unusual dimensions. The Mississippi system, some of whose eastern branches probably date from early Mesozoic time, received great reinforcements by the addition of many long western branches in Tertiary time, roughly contemporaneous with the uplift of the Gulf coastal plain by which the lower trunk of the river was extended to the sea. The present headwaters of that river-trunk to which the name of Mississippi is applied, and which for that reason have gained an undue subjective importance, are of relatively modern date, as they are controlled by the abundant glacial deposits of northern Minnesota. The evolution of the Mackenzie resembles that of the Mississippi in a very general way, although some of its eastern branches may be the descendants of ancestors more ancient than those flowing westward from the Appalachians; but the regime of the great northern river is strikingly unlike that of its still greater southern analogue on account of its course being from a warmer to a colder climate: hence ice-dams, obstructed discharge, and oveflows recur every spring. The Nelson and the St Lawrence systems, draining eastward to Hudson Bay and St Lawrence Gulf, receive drainage from areas that would belong to the Mackenzie and the Mississippi systems under a simpler plan of continental growth; and there is much reason for thinking that this simpler plan obtained until the occurrence of those changes, in association with the Glacial period, whereby sea waters gained access to the depressions that now hold the bays and sounds of the north-eastern coast. In exemplification of the rule that the larger ocean receives the drainage of the smaller continental area, the rivers that flow into the Pacific rank below those belonging to the Atlantic. The greatest is the Yukon, of farther Canada and inner Alaska, one of the great rivers of the world, little known until the active exploration of its basin for goldfields. The Frazer drains much of the mountainous area of southern British Columbia, as the Columbia drains that of the north-western United States; the latter is peculiar in that one of its headwaters rises at the eastern base of the Rocky Mountains in northern Montana and flows westward through the ranges. The Colorado discharges a muddy current into the Gulf of California; but for the aridity of its large drainage area its volume would be much larger. The same is true of the Rio Grande, whose name would be better justified if so much of its basin were not semi-arid.
The most remarkable lacustrine region of the continent, rivalling that of Central Africa, forms a belt around the border of the Laurentian highland; here, in addition to ten large lakes, thereLakes. are hundreds of medium size, and many thousand small lakes. They are peculiar in occupying a region of moderate relief, in which no strong dislocations have taken place in recent geological time (unless in the case of Lake Superior), and thus in contrasting with the great African lakes which occupy rift-valleys or graben of comparatively recent fracture. The Laurentian lakes are further characterized by an intimate association with the ice-sheets of the Glacial period; but while glacial erosion and drift obstruction suffice to account for the smaller lakes, it is very probable that broad crustal warping and drainage reversal have been potent aids to the other processes in producing the great lakes. The northern Cordilleran region contains many beautiful lakes of moderate size in deep valleys among the crowded ranges of the narrowed mountain belt. Their origin has not been closely studied. The basins among the spaced ranges of the middle and southern Cordilleras, in the United States and Mexico, contain many lakes that occupy shallow depressions in desert plains; they are usually without outlet and saline; many of the basins were formerly occupied by lakes of much greater size, some of which overflowed, implying a climate moister than that of to-day, probably correlated with the glacial climate of the regions farther north. Lakes in volcanic craters or behind volcanic barriers occur in Central America, while Florida possesses many small lakes in limestone basins. The following table is taken from Russell's Lakes of North America:—
|Ontario . . . . . . .||247||7,200 (?)||738|
|Erie . . . . . . . . . .||573||9,900||219|
|Huron . . . . . . . .||582||22,322||750|
|Michigan . . . . . .||582||21,729||870|
|Superior . . . . . .||602||31,000||1008|
The climatic features of North America are best appreciated when considered as exhibiting modifications of those general climatic conditions which prevail in consequence of the globularClimate. form of the earth as a whole. In January, when the isotherms of 65° to 75° F. stretch almost directly across land and sea in the north torrid zone, a mean temperature of zero or less invades the region north-west of Hudson Bay, which thus resembles north-eastern Asia in departing greatly from the mean prevailing in similar latitudes on the northern oceans, and in bringing upon the northern lands an extension of frigid conditions that have no analogue in the southern or oceanic hemisphere. In July, when the isotherms of 40° and 50° have a tolerably direct course around the latitude circles that border the continent on the north, a great middle area of North America becomes warmer than the seas on the east and west, having a mean of over 80°, and in part over 90°. In January the Hudson Bay region is 30° colder than the mean of its own latitude, about 60° colder than the mean of the corresponding southern latitude; while in July the Arizona-Mexican region is 20 above the mean of its own latitude, or about 40° above the mean of the corresponding southern latitude. In both winter and summer the isotherms are more closely crowded while crossing the continent than while crossing the adjacent oceans; or, in other words, the poleward temperature gradient is stronger on the land than on the oceans; and all these features should be regarded as inherent characteristics of the climate of North America in virtue of its being a continent chiefly in temperate latitudes.
An associated feature of continental climate is found in the strong annual range of temperature of the central land area. The range between the means of January and July exceeds 40° for the largest part of the lands, and 70° for much of the northern lands; the range of extreme temperatures is much greater. On corresponding oceanic areas in the northern hemisphere the range is little more than 20°, and in the southern hemisphere it is probably less than 10°. It must appear from this that if the largest part of North America is said to be in the north temperate zone, “temperate” must be taken as having little of the meaning originally given to it in southern Europe, for the winter cold is severe and the summer heat is excessive over much of the North American continent.
The several members of the terrestrial wind system, including therein the trade winds of a broadened torrid zone, the stormy westerly winds of middle latitudes and the irregular winds of the polar regions, are well exemplified over North America; but, as is usually the case on land, the systematic movement of the atmosphere is better seen in the drift of the clouds than in the movement of the surface winds, which are much modified by the changes from hill to valley, from mountain to plain. Nevertheless the prevalence of the general atmosphere currents has much to do with the control of certain values of annual temperature range, as well as with the distribution of rainfall. The former are small (about 20°) along a great stretch of the Pacific coast, even as far north as Alaska, where the moderating influences of the ocean are brought upon the land by the westerly winds; while a range appropriate to a continental interior (30° or 40°) is experienced over most of the eastern side of the continent in temperate latitudes, and even upon the North Atlantic ocean near the American coast, where strong seasonal changes of temperature are carried forward by the westerly winds. It is particularly in this respect that the general climatic resemblances between North America and Eurasia, above referred to, are broken; for eastern Canada and western Europe are strikingly unlike in seasonal variations of temperature. Labrador is about 10° cooler than northern Germany in July, but nearly 40° colder in January.
The distribution of rainfall is in general controlled by the prevailing course of the winds. The West Indies receive abundant rain from the passing trades. In Mexico and Central America the eastern slopes are for the most part better watered than the western, because the winds there come chiefly from the east (maximum over 100 in. in Guatemala and adjacent parts). Farther north the reverse holds true; the Pacific slope north of 40° latitude has an abundant rainfall (maximum over 100 in.), and its mountains are clothed with dense forests. There are large areas of deficient rainfall (less than 20 in.) in the interior of the continent, where the intermontane basins and the piedmont plains that slope eastward from the Rocky Mountains in middle latitudes are treeless. The areas afflicted with dryness are unsymmetrically distributed, being west of the medial meridian (95°), because of the ranges near the Pacific by which rain is withheld from the basins and from the plains farther east. The dryness is induced not only by light precipitation, but also by active evaporation in the warm season—a rule that holds true until a high latitude is reached. East of the medial meridian great profit is received from the warm and moist winds that are drawn inland from the water surface of the mediterraneans (Gulf of Mexico and Caribbean Sea) which so advantageously occupy the latitudes that are given up to the Sahara in the Old World. It is largely on this account that the central and eastern parts of the Mississippi basin enjoy a sufficient and well-distributed rainfall, producing forests or fertile prairies over great areas (rainfall over 40 in.). Regions of prevailing snowfall are chiefly in the north-west and north-east; the former includes the higher ranges of the western highlands in Canada and Alaska, where the snowfall from the Pacific winds is heavy, and extensive snowfields and glaciers are formed; the former includes Greenland, where a heavy ice-sheet shrouds the land, the snowfall of moderate measure being probably supplied mostly from the North Atlantic. In the northern continental interior snow covers the ground during the winter season, not that the snowfall is heavy but that the persistent cold weather preserves the moderate amount that falls.
The extension of the continent across the belts of the terrestrial wind system tends to turn branch winds from the westerlies toward the trades on the Pacific border, and from the trades toward the westerlies on the Atlantic border. This effect is strengthened in summer, when the higher temperature prevalent over the continent causes the air to flow away from above the lands, and to accumulate over the neighbouring oceans, on each of which a vast anticyclone is thereby established—the circulation of the atmosphere over the North Atlantic and North Pacific thus coming to simulate the circulation of the surface waters of the oceans themselves. It is partly on account of this deflection of the summer winds up the Mississippi valley that the eastern interior of the continent receives a beneficent rainfall as already stated. In winter when the inflow from the south is replaced by an outflow, little rain or snow would fall but for the indraft winds of cyclonic storms by which the outflow appropriate to the cold season of the continent is temporarily reversed. The free play of the cyclonic winds north and south over the great medial plains permits indrafts from torrid and frigid sources, which sometimes succeed each other rapidly, producing abrupt and frequent weather changes. Something of the same contrasts is produced by winds drawn in upon the eastern coast alternately from over the moist and warm waters of the Gulf Stream, and from over the moist and cold waters of the Labrador current.
The southerly flow of the branching winds along the Pacific coast gives them a drying quality, and thus still further broadens the western arid region towards the ocean until it reaches the coast in southern California and north-western Mexico (rainfall less than 10 in.), there joining the arid belt of western Mexico and presenting a strong contrast to the rainy forested coast farther north; but although unfavourably dry, the southern California coast is one of the most truly temperate regions of the world, in respect of mildness and constancy of temperature. The drying winds cover all California in summer, but they migrate southward in the winter, giving place to the stormy westerlies. Thus California has a subtropical climate of wet winters and dry summers; while north in British Columbia and Alaska there is plentiful rainfall all the year round, and farther south there is persistent aridity.
The fauna of North America (Nearctic) is more closely related to that of Europe-Asia (Palaearctic) than to that of any other zoogeographical province; the two being united by manyFauna. writers in one faunal province (Holarctic). The reindeer (caribou), beaver and polar bear are common to both provinces. The moose, wapiti, bison and grizzly bear of North America are closely related to the elk, red deer or stag, aurochs and brown bear of Eurasia; and the following groups are well represented in both provinces: cats, lynxes, weasels, bears, wolves, foxes, seals, hares, squirrels, marmots, lemming, sheep and deer. On the other hand the following forms are characteristic of North America: (rodents) pouched rats or gophers, musk rat, prairie dog, Canadian porcupine; (carnivora) raccoon and skunk; (ungulates) musk ox, bighorn, Rocky Mountains goat, pronghorn; (marsupial) opossum. Among birds there is a close resemblance to those of Eurasia, with some admixture of South American forms, as in the humming birds. The forms especially characteristic of the northern continent are the Baltimore oriole, bobolink, cowbird, flycatchers, wood-warblers, Californian quail, tree grouse, sage grouse, wild turkey and turkey buzzard. The house sparrow of Europe has been introduced, and has become very common, especially in the cities, where it is known as the English sparrow. Reptilian and amphibian groups are well represented; turtles are especially numerous; salamanders are varied and large; rattlesnakes are among the more peculiar forms. Among fish, the characteristic forms are the cyprinoids (carp), sturgeon, salmon, pike and especially the suckers, sunfish, mudfish (Amia) and gar pike (Lepidosteus). The most characteristic group of invertebrates is the Unionidae or river mussels.
The floral areas of North America, limited by the geographic divisions of the continent, may be divided into five belts: the eastern forested area, the western forested area, theFlora. interior unforested area, the northern barren lands and the Gulf coast. The eastern forested area extends from the Laurentian highland in Canada to the Great Lakes, and southward east of the Mississippi to the Gulf coast. In the north and along the mountains southward, the forests are largely coniferous, with a mixture of birches, poplars and maples. Southward, especially in the interior and at low altitudes, the conifers largely disappear, and oaks, hickories, plane-trees, tulip-trees, walnuts and other valuable deciduous species abound. The western forested area begins in the eastern Rocky Mountains and extends to the Pacific. Eastward in the mountains the forests are interspersed with arid districts which increase in area southward. Northward, in Canada, the mountains of the middle Cordilleras are densely wooded with continuous forest up to the timber line. Near the middle Pacific coast the forests attain a luxuriant development, the redwood (Sequoia) of California and Oregon sometimes reaching a height of from 300 to 400 ft. The unforested area of the interior consists of two very dissimilar portions. The vast fertile prairies extend from the Great Lakes westward to the Great Plains, and southward west of the Mississippi, with occasional eastward lobes at low altitudes. On these plains grasses and other herbaceous vegetation abound, and throughout this fertile belt agriculture is largely followed, the grain and hay crops being especially important. Northward in Canada the plains become wooded, the western mountains and the eastern highlands being thus connected by a narrow strip of forest. South-westward and westward the fertile prairie gives way to a vast arid region beginning on the Great Plains and extending as far as south-eastern California, and thence southward into Mexico. On this broad desert few trees are found, although piñons grow on the cliffs and ledges, and cottonwoods occur along the watercourses; but the various ranges that surmount the desert frequently carry forests. The desert vegetation as a whole consists of cacti, agaves, sage-brush (Artemisia) and other plants adapted to arid conditions. North of the eastern forested area and east of the northern Cordilleras are the “barren lands,” with frozen subsoil, extending to the Arctic coast. The growing season here is short and the climate forbidding, so that trees cannot develop, although birches, poplars, willows and other genera, which southward attain large size, are present as dwarf shrubs. The vegetation of this northern barren district, like that of bleak mountain summits southward, is very similar in character to that of other extreme boreal regions. Blueberries, crow berries and some other small fruits are abundant, but the brief summer will not mature most crops of the temperate zone. The Gulf coast, on the other hand, supports a vegetation decidedly tropical in its nature. Somewhat developed in Florida and the other southern states, this flora becomes the prevailing one on the coast of Mexico and Central America, especially from the region of Vera Cruz southward, where the forests are largely composed of palms and live oaks, and where giant bamboos often attain a height of 40 ft. In these tropical forests many orchids and other showy plants of northern conservatories are native.
North America, with an area of about 8,000,000 sq. m. (16% of all the lands, or 4.12% of the whole earth's surface), and a mean altitude of about 2000 ft., at present, p1ays a part in human history that is of greater importance than is warranted by its size alone, although it has not in this respect the extraordinary importance of Europe. The continent has the Economic Development of the Continent.good fortune to lie chiefly in a temperate rather than a torr1d zone, and in temperate latitudes to be much nearer to Europe than to Asia. Whatever may have been the first home of the aboriginal inhabitants, the dominating people of to-day are derived from the leading countries of the Old World. Not only so; temperate North America has become the most progressive part of the continent because of receiving its new population chiefly from the most advanced nations of middle western Europe—Great Britain, France and Germany; while the torrid islands and the narrowing southern mainland of North America have been settled chiefly from the less energetic peoples of southern Europe; and the inhospitable northern lands are hardly entered at all by newcomers, except in the recently discovered goldfields of the far north-west. From the plantation of colonies on the eastern coast, the movement inland has been governed to a remarkable degree by physiographic factors, such as form, climate and products. The cities of the Atlantic harbours and of the adjacent lowlands still take a leading part in industry and commerce, because of their longer establishment and of their relation to Europe. The uplands, ridges and mountains of the Appalachian system—the “Backwoods” of a century ago—remain rather thinly occupied except at certain centres where coal or other earth-product attracts an industrial population. Beyond the Appalachians the middle interior contains a very large proportion of habitable land. It was long ago recognized as a land of great promise, and it is to-day a land of great performance, covered with a network of railways, yielding an enormous product of grain, and developing industries of all kinds. Indeed, within and closely around an area marked by the St Lawrence system on the north, the Ohio on the south, and stretching from the Atlantic coast between the Gulf of St Lawrence and Chesapeake Bay inland to the middle prairies, there is a remarkable concentration of the population, industry, progress, wealth and power of North America—the focus of attention from all other parts of the continent. The regions of the far north and north-east, including the greater part of the Laurentian highland and the extreme northern stretch of the medial plains and the western highlands, remain and will long remain thinly populated. The furs of wild animals are their characteristic product. Timber is taken from their more accessible forests; but only in mining districts does the population notably increase, as in the iron region around Lake Superior and in the Klondike gold region.
In the south-eastern United States lies a belt of coastal lowlands skirting the Appalachians, still affected by negro slavery and its consequences. The descendants of the early French settlers of Canada stand in political rights as well as in loyalty to the Government on an equal footing with the British citizens of the Dominion. The Italians of the cities, the Hungarians of the mines, the Scandinavians of the northern prairies, the Irish and Germans everywhere are “Americanized” in the second or third generation, rapidly entering local and national politics, and hardly less rapidly attaining an honourable social standing as tested by intermarriage with English and other stocks. But the negro is set aside, even though he has adopted the language and the religion of his former masters: political and social rights are denied him, and intermarriage with whites is practically excluded, although illegitimate mulattos are numerous. Thus has slavery left upon a people, amongst whom political rights and social opportunities should be equal for all, the heavy burden that always retards progress where strongly contrasted races are brought together. Farther south still are the tropical islands and the narrowing mainland, rich in possible productiveness, but slowly developed because of a prevailing diversity and instability of government and lack of progressive spirit among the people. Here also there is a considerable proportion of negroes, but they live under less unhappy conditions than those now obtaining in the United States. In Mexico and Central America, the proportionate number of aborigines is much greater than farther north.
West of the Mississippi in middle latitudes the population rapidly decreases in density, and over a large extent of the semiarid plains it must long remain sparse. The settlements bordering the plains on the east for a long time marked the “Frontier” of civilization, for the vast stretch of dry country was a serious barrier to farther advance. But the plains are now crossed by many railways leading to the Cordilleran region—the “Far West”—in large part too rugged or too arid for occupation, but rich in minerals from one end to the other, the seat of many mining camps of unstable population, and containing numerous permanent settlements in the intermontane basins. Great irrigation enterprises, conducted under the National Reclamation Service of the United States, are employing all available water supplies for agriculture; but large areas must remain permanently desert. On nearing the farther ocean the climatic conditions improve, and the population is rapidly increasing in number and wealth; this district not being content to take its name with respect to the east, not considering itself as included in the “Far West,” but choosing the distinctive designation of the “Pacific Slope,” and, while maintaining an active intercourse all across the breadth of the continent, already opening relations with the distant Orient by a new approach. Among the earliest results of the latter movement was the arrival of Chinese labourers, a humble, industrious and orderly class of men, but one which stands apart in language, religion and race from the dominant population, lives largely without domestic ties, and gains neither political nor social standing in the New World.
Two centuries ago the aboriginal population of North America would have deserved description before the immigrant population. To-day the aborigines are displaced from nearly all the valuable parts of the continent. Never very numerous, they are now decreasing; many tribes are already extinct, many more are almost so. Those which remain less diminished are in the Far North or North-West where nature is rigorous; or in the tropical forests of Central America where nature is over bounteous; or in the more desert parts of the Middle West where nature is arid. The replacement of the native races by the foreign has too often been harsh, cruel and unjust; yet it has resulted in an advance of civilization. Many savage tribes, speaking many different languages, holding little intercourse with each other, and frequently engaged in intertribal wars, have given place in little more than two centuries to a great population of European origin, whose dominant parts speak one language, whose arts are highly advanced, whose home intercourse is most active, and whose foreign commerce had attained unexpected proportions at the opening of the 20th century. (W. M. D.)