Popular Science Monthly/Volume 19/June 1881/The Primeval American Continent
|THE PRIMEVAL AMERICAN CONTINENT.|
THE reader may recall standing, when a child, by the side of a toy-dam in the course of some little stream, and, were a breach made in the mimic masonry, remember the mute interest with which he watched the slow emergence of fairy islands as points of rock and shoals of mud slowly appeared above the water's surface—how the detached summits, at first round spots, assumed varied outlines indented with cones or bristling with promontories; how they multiplied as the ebbing water exposed newer and lower levels, until the tiny sea was dotted with an archipelago of islands, whose nearing shores, gradually joining, formed chains of islets; how the inclosed area of water contracted, and, in the union of all their separate figures, vanished. The wet surfaces, broken by depressions, were marked by pits of water from whose sides stole along intermediate creases, threadlike lines of water, the river system of the miniature continent, its sinuous shores impressed by the ripples of a mimic sea. The recollection of boyish pleasures becomes touched with a deeper interest when one is taught to recognize in this the picture of what in a larger way, and under cosmic conditions, has happened in the life-history of our own sphere, and to realize that his childish hand may have reproduced at will a somewhat exact copy of the stages of the world's growth.
Suppose this little world so briefly made had been left till the bright sunshine had dried its surface, in some places parched and cracked it, in others evoked a luxuriant growth of grass and flowers, filled its shores with gliding snails and its level mead with teeming ant-hills. Once more the running stream is stopped, and slowly the muddy tide rises higher and higher, obliterates the lowlands and creeps slowly up the sides of the high ground, around the skirts of its pygmy mountain-chains, and rolls restored amid groups of remade islets. Under the guidance of this retrospective knowledge let the current be arrested to permit some portions of that first-made land to remain uncovered. With favorable conditions, as a stream carrying abundance of fine silt, the water moving sluggishly over the inundated surfaces will drop its earthy burden upon them in thin, loose layers, evenly at first, but, as shallows are formed, irregularly modified, or as sudden freshets produce stronger currents, eddies and ripple-bars gather or disturb the sediment. New lands in wet banks and knobs will begin to appear over the highest ridges. Break away again the barrier, and cause the water to recede, exposing in a similar succession of phases the bared surface. The expectant eye now notices certain changes: old landmarks are removed from sight, valleys have been converted into plains, new hills have arisen, lakes are seen here and there where before was dry land, new contour lines surround the structural ridges which yet remain, modified indeed, but distinctly recognizable, while the low verdure of grass and the numerous pinnacles of ant-hills have together disappeared beneath a universal blanket of mud. Only upon the very highest points do the scattered remnants of the first surface appear unchanged, because they alone were exempted from general flood. Again the sun shines upon it, new seeds sprout, other insects flourish, and fresh showers fall. In spots the old land is exposed; the new having been washed away from it; it may be easily recognized by its fauna and flora, the drowned ants, and the hidden grass.
Again and again, under varying conditions, may our experimental world be sunk and raised, preserving, possibly, through all the changes the rudimentary outlines of hills and valleys by which it was first characterized and marked. And imagining that every new stratum was ingeniously varied in the color and to some extent in the nature of its mineral constituents, and that, upon each reappearance of this diminutive continent, the skillful experimentalist spread a new form of life, what at last might be, after some exposure to weathering and change, the character of its surface? Evidently something like this: in the first place, since, in consequence partly of denudation and partly of only partial submergence, certain tracts will appear cleared of the later deposits, we will find numbers of the whole series laid bare in spots from the highest point, where, as presupposed, only the primitive layer is seen, to the outskirts of the island where the latest layer forms the surface; or to interior depressions, where lake-like centers existed alternately filled and emptied with each recurring deluge. Endless diversity might be introduced into such a scheme, so far as local detail is concerned, but under the conditions given the island would exhibit lines of stratification, each distinguished by color or fossils, and following each' other in the order instituted by the youthful world-makers. Along the crevices and tiny gullies we might detect the minute succession of various strata, and at intervals fragments of the buried life would be revealed. Enlarge this minute illustration till it assumes continental dimensions, reverse the periodic inundations from the water rising to periodic inundations from the land sinking, and in a rude way, subject to important modifications, the reader will be prepared to realize the formative system developed in the construction of our northern hemisphere.
Through a sequence of phases, somewhat distinctively bounded by periods of depression and consequent submergence, and periods of elevation and consequent drainage, land was added to an initial nucleus by enormous marine accumulations, the débris of animal organisms, and the detritus from terrestrial abrasion. Chemical action, heat, and molecular transference hardened these layers into stone, and thus the new-made land, though undergoing change from recurring submergence, and through subaërial denudation, yet, to a great extent, resisted removal while it contributed to the growth of the incipient continent, and formed the ground upon which new-laid strata were heaped.
In the American Museum of Natural History, seven maps, the work of Professor R. P. Whitfield, have recently been added to the Geological Cabinet, intended to exhibit the growth of the eastern half of the North American Continent from 95° west longitude eastward to its shores. The scheme of their arrangement is the exhibition, by contrasted colors, of the superficial areas where to-day the rocks of the various geological epochs are exposed, beginning with the oldest and rising to the youngest, whereby we seem to seize at critical points the stages of the continent's enlargement, and follow with the eye the stupendous changes which shaped it.
The first chart presents in its colored parts the primeval territory, which geologists regard as the first-made land of our continent, the archaic regions, around whose rocky framework were gathered the accretions of succeeding ages. It is the azoic terrain, that composite foundation of gneiss, granite, schists, crystalline limestones, sandstones, serpentine, and iron-ore masses, which defined the geological architecture of America. In its isolated ridges, cleared of the later and adjacent strata, we have before our eyes the principal portions of a continent upon which the ancient oceans played
"Of pure ablution round earth's lifeless shores."
Its sterile stretches unalleviated by a mantle of waving woods, unanimated by moving figures, reflected the harsh sunshine from rugged terraces or monotonous lowlands, a cheerless waste bathed by preadamite seas.
Starting from a point near Montgomery, in Alabama, the archæan country stretches northeasterly along the Appalachian axis and, rapidly widening, incloses large districts in Georgia, western South and North Carolina, of which latter State it defines the western boundary, and reaches eastward nearly to Raleigh. Passing on both sides of a lenticular area lying in North Carolina and Virginia, it narrows to a strip west of Richmond, where it is deeply bitten by a round gulf, and pressed to the seaboard, forms a thin isthmus west of Washington, then expands at Baltimore, and, lobed out into a pennant-shaped appendage, reaches down toward Newcastle, Delaware. From a little west of Burlington, nearly to Easton, a white patch shows an area where the archæan rock is no longer seen, but at the latter point a thin strip follows the Appalachian uplift and, including the highlands of West Point, appears as an attenuated finger or arm of a great area, which pushes south as far as Manhattan Island, whose gneissoid rocks compose it, and eastward over the western half of Connecticut. In Massachusetts the archæan rocks bifurcate; a finger reaches to the northern boundary of the State, where a thin connection exists with the great eastern region, and a shrunken area extends northward through the Berkshire Hills. The western limit of this latter strip lies some ten or fifteen miles from the eastern boundary of New York, and, entering Vermont at its southeastern border, widens out till, at Montpelier, almost half the State is covered. Slowly broadening thence, we follow its outlines into Canada, approach the St. Lawrence, and then, with an abrupt eastward deflection, trace it in a sinuous tongue until it touches the river at Mount Camille. The large eastern seaboard area of archæan rocks commences at Saybrook, on Long Island Sound, whence northward, limited by a sweeping curve, it covers the eastern part of Connecticut, almost all of Rhode Island, and eastern Massachusetts, with some slight exceptions, where islands of later rock occur, as southwest from Boston and about Lowell. Nearly all New Hampshire is covered by it, and in Canada it forms another strip parallel to the first, while eastward it constitutes the surface rock of much of Maine, wherein, at last, it breaks up into scattered patches, lying like Titanic stepping-stones, from Augusta northward to the desolate horn of Newfoundland. One of these districts surrounds Mount Katahdin; another, in a long, easterly-deflected strip of land, runs from Mount Desert northward to Chaleur Bay, New Brunswick; while from Machias Bay a third streams northward in a narrow ribbon.
Separated areas are found along the southern shore of Newfoundland and upon Cape Breton Island.
In the United States four other extensive archæan territories exist east of 95° west longitude; one in the Adirondack region, embracing the immense northern park of New York as far north as Malone, and stretching southward almost to Saratoga Springs, bordered by the State line, and, linking, through a narrow aperture between surfaces of subsequent strata, with the enormous reaches of azoic land which form Quebec and Ontario Provinces, it merges into two lateral expansions, on one side into the limitless highlands of Labrador, on the other into the ridges, valleys, and plateaus of the lake country northward to the Arctic Circle.
The second area is in northern Wisconsin and Michigan, embracing the Marquette region, famous for its ores. The third is a neighboring and related province in eastern Minnesota, from the South Bend on the Minnesota River, widening northward and uniting with the Canadian area about the Rainy Lake region. The fourth, a diminutive outlier, comprises the Iron Mountain and Pilot Knob country in southeast Missouri.
These large spaces of archæan rock represent the floor-layers, as now exposed in the eastern United States, of the continent's superstructure. In these parts of our country they form the surface-rock, and whether they have been always raised beyond the reach of sedimentary deposit or have been scoured and relieved by frost and flood of superincumbent strata, whether their present extent is conterminous with their limits, as once revealed above the level of primeval seas, or whether shrunken by subsidence and partially obliterated by later formations, they are at any rate outcrops of the vast bedding on which ocean and continent alike repose. But when we examine these aged stones we find that they themselves appear as the cemented residues and stratified deposits formed from some yet preëxistent firmament of land. In serial bands, conforming to each other, as book lies against book, we find limestone, slates, sandstones, quartzites, schists, and gneiss, and we know now that these regular layers, hard, distinct, and characterized by color, constituents, and adventitious minerals, were once water-drifted beds metamorphosed, transfigured, as it were, by heat and pressure into this adamantine pavement; and, further, we find that they must have been so formed in the attrition and decay of yet older continents. The dim perspective opens backward to the very verge of chaos.
After deposition, and in a somewhat consolidated state, they were slowly raised, their emergence above the water accompanied a contraction of the earth's crust, and the flexible series, from top to bottom, folded up in deep and multiplied plications. Mountain-chains arose, their strata tilted up, contorted and complicated in related groups of synclinal and anticlinal axes, and, by the effective agency of heat and aqueous distillation through the myriad pores of the rock, a mineralogical change ensued. The argillaceous muds were hardened into slates and schists, the calcareous shoals became crystalline limestones, marbles, and dolomites, the siliceous bands became quartzites and sandstones, the iron slime crystallized into colossal sheets of iron-ore, magnesian sediments became serpentine, and through all there developed beautiful minerals under various associations and marking different horizons in this complex pile of natural masonry. Feldspars, pyroxene, mica, apatite, chondrodite, epidote, and garnet are a few of many which, in crevice and seam, and scattered through the matrix rock, remain as token, and possibly revelation, of the changes here enacted. From this archæan country come the magnetic oxide of the Adirondacks, the hematite of Marquette, the soft lead of Ticonderoga, the dolomite of Westchester County, the mica of North Carolina, the syenites and granites of Maine, the marbles of Vermont, the tinstone of New Hampshire, and the phosphates of Canada. Over thirty thousand feet in vertical thickness is the estimated depth of this gigantic mass—fitting foundation for the arches of the world.
Recent study, notably that of Dr. T. Sterry Hunt, separates this wonderful epoch into four secondary ones of unequal duration and varying character. First, the Laurentian, a name given by the Geological Survey of Canada and applied originally to the rocks of the Laurentian highlands, those abraded swells of land which overlook the St. Lawrence and rise in rugged grandeur four thousand feet high above the shadowed waters of the Saguenay. This primitive tract of archæan territory embraces the Adirondacks of New York, the region about Ottawa, portions of Newfoundland, and probably includes the rock assigned to this age in Massachusetts, Connecticut, and Rhode Island, and the long back which makes up the Highlands of the Hudson, the South Mountain of New Jersey, and the ridges about Richmond and Mount Roan in North Carolina. The rock is "a strong, massive gneiss, reddish or grayish in color."
Following this is the Norian, unconformable with the Laurentian, viz., not fitting into it, as though the latter, first made under water, solidified and raised, had again been depressed and received these secondary deposits. The Norian rock is distinguished by the abundance of labradorite, a feldspar possessing iridescent tints, and is found in Essex County, New York, Labrador, extensively along the St. Lawrence, upon Lake Huron, while "bowlders of it are occasionally found along the eastern shores of Maine and Massachusetts, and also in northern New Jersey."
The Huronian era succeeds, and is a name applied to the upper layers of the Huron Mountains, Lake Superior, to the Green Mountain series, and to detached areas along the coast of Newfoundland, eastern New England, and southward upon the flanks of the Blue Ridge. The Mont Alban series marks the fourth period, so named after the White Mountain layers in New Hampshire, where the aggregated display of crystalline schist is assigned to this province. New York, Philadelphia, Baltimore, and Washington occupy this terrain, and these rocks occur throughout the Blue Ridge, as far as Georgia, of more than passing significance, as they form the gold-bearing strata ia Virginia, North and South Carolina. In these rocks the garnet, staurolite, cyanite, and chiastolite, favorites of the mineralogist, are almost exclusively found.
Instinctively we ask: Did no living thing exist through all these ages; did the mechanical wear and tear of rock-masses and their redeposition by mechanical means solely occupy the desolate centuries? The proofs of organic activity, involving the functions of life, are numerous, but the exact character of that life and the special conditions under which it flourished are greatly if not entirely wanting. In the first place, we find in Canada important, indeed inexhaustible deposits of carbon under the form of graphite, and graphite occurs in our coal measures as the direct product of alteration from coal. These huge masses, distributed in pockets, sheets, and nodules through the archæan rock, indicate the presence of vegetative forces, doubtless exhibited in plants of a low order, but on a scale of tropical exuberance.
These carbon pockets occupy the shrunken areas of what were once vast, waving, and deeply matted beds of algæ, sea-weeds, building up, through innumerable generations amid the gathered detritus of shore and cliff, dense piles of carbonaceous remains. Or else they are attributable to a fertile growth of lichens which spread, possibly with an almost arborescent vigor, over plain and mountain. These organisms are low in the vegetable hierarchy, and along with them may have lived allied families: the microscopic Desmids and Diatoms, whose siliceous tests showered down through the still oceans; beside them the Corallines and Nullipores, forming calcareous fringes and coral-like thickets; the minute Protophytes and the delicate Charæ. Doubtless this age marked the climax of these plants, and, through multiplied species and in vast numbers, they represented one phase of the ever restless evolution of vital forces.
The great deposits of iron-ore, though affording no direct evidence in their remains of plant-life, are no less trustworthy proofs of its existence. They are themselves largely the result of vegetation in some form. Dr. Hunt originally explained this connection, illustrating it by identical processes in the world about us. If the reader visits a bog-land in summer, where slowly-running or stagnant water collects in pools, or if he stands upon the edge of a morass or marsh, he will notice angular, iridescent films floating upon the surface. They are thin pellicles of iron oxide, which will soon break up and sink, to be succeeded by fresh "skins," which in turn disappear, building up a growing layer of bog-iron ore beneath the water. The theory is simple. Iron exists under two forms, a soluble or monoxide, and an insoluble or sesquioxide. The latter is widely disseminated through the rocks and soils. The insoluble modification is reduced to the monoxide or soluble state in the presence of finely divided and rotting vegetable matter, or in water charged with vegetable infusions, as emacerated leaves and tissues. Rains and streams carry it away to lowlands and depressions, where it becomes, through contact with the air, again oxidized or rendered insoluble, and is redeposited in streaks and bands. The widespread action of vegetable acids is here concerned. Humic, crenic, apocrenic, and related acids, in conjunction with the reducing power of carbonaceous residues, removed iron oxide from the original rocks, and through the agency of water gathered it—useless as long as it remained scattered in minute particles through vast terrains—into enormous masses, the source and maintenance of our industries, thus garnered through these gentle and silent methods. Such has been the growth of the large deposits in the Marquette region, in the Adirondacks, and at Pilot Knob—deposits which under the influence of heat have become changed into the specular ores, the magnetites, and hematites. They point unmistakably to the existence of plants, and no less to their duration over immense periods of years.
The proofs of animal life are less satisfactory, and have been discredited in high scientific writings, or, more accurately, the morphological types of that life have been rejected, leaving the general presumption unquestioned that animal life of some kind prevailed. In the first place, the phosphatic minerals found in the archæan rocks are considered derivative from organic remains, as to-day phosphorus as a phosphate results from animal secretions, though phosphorus is omnipresent in the plant-world, and the ashes of various vegetables yield from eight per cent, to fifty-three per cent, of phosphoric acid, while the annual shipment of flour and wheat from our shores represents thousands of tons of this element. In this respect the evidence does not seem altogether controlling that these archæan phosphates necessarily resulted from animal débris. But the argument rests upon surer grounds. In 1868, Logan, Dawson, Carpenter, and Hunt, prepared a paper of great merit upon an archæan fossil, which they named Eozoön, and which they considered representative of the zoölogical subkingdom of the Protozoa and allied to Foraminifera. They represent it as an organism attaching itself by a gelatinous body to sea-floors, enveloping itself with a crust of carbonate of lime in which very small tubes penetrated to the surface through which the sarcodous material within projected in tapering fingers, to be withdrawn at the will of the animal; upon this another layer of protoplastic matter, formed in the growth of the creature, connected with the first, but separated throughout most of its extent by an interlamination of limestone, in which radiating canals are discerned, and which succeeds the earlier poriferous shell. Upon this new calcareous crusts arise, and thus a cellular and tuberiferous mound is formed, compacted and regular, along the base of attachment, but loose, granulated, and divergent at its summit.
In our present seas closely related organisms appear, the Rhizopods, minute bodies, structureless, mere pellets of protoplasm, yet possessed of a secretive function which incases them in exquisitely symmetrical houses of lime. They are naturally low in the animal scale, indeed primary, and the Eozoön seems to have been a Titan progenitor of these hosts of later protozoans whose numberless fragments form the chalk beds of England and France.
The Eozoön Canadense is found in the Laurentian rocks of Canada, other species in the Huronian of Bavaria, and specimens have been described from the Adirondacks and from Massachusetts. Forms strikingly resembling Eozoön may be found in the serpentine ledge in Fifty-ninth Street, near Tenth Avenue, New York. The soft parts in the calcareous skeleton of this Rhizopod have been replaced by minerals, and on the resemblance, amounting almost to identity, between the Eozoön and certain mineral pseudomorphs are based the objections made to its acceptance as of organic origin. King and Rowney, of Dublin, and Möbius, of Germany, have very vigorously attacked it, and lately Roemer rejects it from the list of palæozoic fossils. But it seems impossible to doubt the reality of its animal arrangement. Professor Hitchcock thoughtfully observes in this connection as regards its resemblance to mineral replacements, "Inasmuch as these structures represent the higher efforts of the mineral kingdom in crystallization and the nearest approach to the inorganic world allowed by animal forms, it is not strange that the two extremes should resemble each other sufficiently to deceive practical observers."
This was, in a few words, the archæan Continent. Its greatest area was in the north, with scattered islands and thin prolongations southward along the present axes of elevation. Subsequent periods built out from this and filled in the shadowy but prophetic sketch of North America—not an azoic or lifeless country, as once thought, yet a territory where silence reigned, broken only by the roar of the surf along its bleak margins, the whistle of the gale through its defiles, and the thunder of tempests upon its plains. "Lonely, silent, and impassive, heedless of man, season, or time, the weight of the Infinite seemed to brood over it."