^torth]
AMERICA
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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 feet; mountains near the Labrador coast, 8000 feet), again ofiering 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 “ Arcluean 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 “ Archaean 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 “ Archaean 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 Kocky 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 North America. the older geologists to be first rising from the primeval The following sketch of the geological development of ocean, it is now proved to have been slowly sinking after North America considers the larger physiographic divisions a prolonged land existence. The crystalline Archaean rocks in the Laurentian region and its scattered fellows in the order already presented. The extensive area of ancient crystalline rocks (Archaean), cannot possibly be explained as a primitive sea bottom, stretching from Labrador past Hudson Bay to the Arctic rising above sea-level to make the beginning of a continent Ocean, is of greatly disordered structure, and and receiving Cambrian strata upon its still submerged hence must have once had a mountainous form. borders, but only as portions of an already old and deeply** Moreover, the crystalline texture and deformed denuded land area, which was in pre-Cambrian time much foliation of the rocks prove that the surface now seen larger than the visible Laurentian area of to-day, and was once buried deep beneath the surface of an earlier which was reduced to perhaps half its primeval dimensions time, for only at great depths can such texture and by a gradual submergence beneath the transgressing sea foliation be acquired. Both these lines of evidence lead in which the Cambrian sediments were laid down. We to the conclusion that the moderate relief prevalent oyer are thus led to believe that much of the continent of the existing Laurentian region is the work of persevering to-day was a continent in the earliest geological times, and erosion during a long continuance of dry land conditions, that the seas which partly covered it in Palaeozoic and and hence that the region must be regarded as one of Mesozoic time were due to partial submergence, not to those worn-down mountain systems whicly so eloquently partial emergence. Furthermore, all the marine strata testify to the vast duration of geological time. But. the that now stretch over a large part of what is believed to full value of this testimony is appreciated only when it is have been the ancient continental surface are of relatively discovered that the worn-down old land is gently over- shallow water origin; none of them bear any close relapped, chiefly around the south and west, and south of semblance to the deposits of the deep oceans that have Hudson Bay, by very early Palaeozoic strata which rest been so well studied in the last thirty years. Hence the upon the eroded surface of the crystallines, thus proving Palaeozoic and Mesozoic seas of North America were not that the destruction of the ancient mountains had already deep oceans, and as far as this continent is concerned it. is been accomplished before some of the oldest fossiliferous by no means admissible to assume, as some of the earlier formations of the world had been deposited. All the geologists did, that the position of continents and oceans evidence goes to prove that from then to now the Lauren- have repeatedly changed places. The testimony of the tian region has been relatively quiescent, as if the forces rocks is decidedly in favour of Dana’s view that continental of deformation had been exhausted by their efforts in the masses are relatively permanent, and hence also, as far as earlier ages of the earth’s history. In all subsequent stability is concerned, in favour of the theory of a tetratime there have been here no such great crushings and hedral earth. The early history of the Laurentian region has been upheavals as have occurred elsewhere, but only moderate
the most modern periods, that neither can regard the other as older or younger than itself. No simple generalization is admissible concerning the age of anything whose history is so complicated as that of a •continent. The climatic contrasts between North and South America are replaced by 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 central 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 suspected former extension of these lakes in central Asia agrees well with the proved extension of the lacustrine conditions in western North America. In matters of temperature, however, there are certain con trasts, as will be more fully stated below when the question of climate is discussed.
