North America

NORTH AMERICA. The continent of North America has a mean altitude of about 2,000 ft., and an area of some 8,000,000 sq.m., 16% of the earth's land area and 4% of the earth's entire surface. Certain correspondences of its geological structure and history with those of the greater part of Europe and Asia, sym metrically disposed, right and left, with respect to the Atlantic, support the view that Eurasia is a single continent : furthermore they controvert the idea that the New World of discovery is an old world geologically; and that the Old World of history is geo logically new. Both worlds are in part old, in part new; both share structures and changes of similar dates from the most an cient to the most modern periods.

The Laurentian Region.

The general structural features of North America are as f : the extensive area of ancient crystalline rocks, stretching from Labrador past Hudson bay and thence north-westward to the Arctic ocean, is of disordered struc ture ; the surface now seen must have been originally buried at a considerable depth beneath a higher and mountainous surface ; but that higher surface was reduced to low relief at a remote pe riod. The Laurentian part of this great region has been described as the first emerged land area of North America, around which many later additions on the south and west built up the present continent ; but a more modern interpretation shows that the worn down Laurentian region is only part of a much larger land mass which in pre-Palaeozoic time extended more or less continuously as far as Texas and parts of the Rocky mountain region, and on which marine deposits were unconformably deposited as the land mass was progressively submerged beneath the Palaeozoic ocean. Hence the Laurentian region differs from other parts of North America more in having escaped later deformation than in being of earlier origin. In Pleistocene time, the Laurentian region be came the centre from which ice sheets spread out on all sides. As a result the weathered soils were swept away, together with an unknown measure of unweathered rock, leaving scattered boulders and gravelly drift upon a rugged upland without mountains (ex cept in Labrador), but diversified by innumerable knobs and hollows, and here and there covered by clay belts that are arable though of severe winter climate. The drainage of the region, which in pre-glacial times was probably accomplished by well ordered rivers, was thus thrown into great disorder; large and small lakes abound, and some of the lakes have two outlets; the streams are repeatedly interrupted by rapids and not infrequently split into two channels, enclosing islands many miles in length. The region remains a forest wilderness, except at mining centres (several of which have produced large quantities of nickel, silver and gold), pulp mills and on the sparsely settled clay belts.

The Appalachian Area.

This is a hilly and mountainous belt, stretching from Newfoundland to Alabama, with a western branch in Arkansas. It seems to have belonged in the earliest times to the great pre-Palaeozoic land mass, but it must be set aside from the undisturbed Laurentian region because of repeated movements of depression, deformation and elevation that it has suffered, generally along a northeast-southwest trend, causing alternations of heavy deposition and almost equally heavy degra dation. These movements took place with generally decreasing in tensity, through nearly the whole stretch of geological time cov ered by the fossiliferous record. The Appalachian mountains of today were formerly regarded as unconsumed residuals of greater mountains formed at the close of the Palaeozoic period ; but it is now generally agreed that Mesozoic erosion reduced most of that ancient 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 to-day, as well as of a belt of horizontal strata on the west, now known as the Allegheny plateau, are largely the result of Ter tiary elevation and dissection of the previously worn-down mass; the additional height thus given to the subdued mountain groups made them the loftiest parts of the range to-day, as in the White mountains of New Hampshire (Mt. Washington, 6,295 ft.) and the Black mountains of North Carolina (Mt. Mitchell, 6,711 ft.). The Ozark plateau of Missouri and the Ouachita mountains of Arkansas and farther west are related to each other in much the same way as the Allegheny plateau and the middle and southern Appalachians. Numerous coal seams occupy discontinuous basins in the Appalachians from Nova Scotia to Pennsylvania and in the Allegheny plateau from Pennsylvania to Alabama, and in the extension of the same strata across the Ohio basin. The eastern coast of the continent has a rocky and irregular shore line from Greenland and Labrador to Massachusetts, with numerous sub merged valleys forming bays and as many uplands and ridges out standing in promontories and islands; this being the result of an increasing measure of depression to the north, where an archi pelago now replaces what was probably once a corner of the conti nent. A coastal plain of gently inclined and imperfectly consoli dated strata, which still borders the Gulf and part of the Atlantic coast of the United States formerly extended north-east, prob ably at least as far as Nova Scotia; the same depression that has brought the ocean upon the older rocks from Massachusetts northward, has diminished the breadth of the coastal plain and embayed its shore line from North Carolina to the mouth of the Hudson, and has submerged it from the Hudson mouth, north eastward.