Page:1902 Encyclopædia Britannica - Volume 25 - A-AUS.pdf/405

 north]

AMERICA

denuded Archaean 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 mountainmaking 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 early 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 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 feet), and the Black Mountains of North Carolina (Mount Mitchell, 6711 feet). 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 Allegheny plateau (altitudes, 2000 to 4000 feet). It should be added that the Osark 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 Allegheny plateau and the middle ranges of the Appalachians the two pairs corresponding to a remarkable degree in regard to conditions of ancient accumulation, mediaeval 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 worndown Appalachians of Nova Scotia, Pennsylvania, and Alabama, as well as in the Allegheny 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 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

359

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. 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 United States and Canada), differ from the Laurentian and Appalachian No^S ° regions in having suffered numerous disorderly America. 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(40,000to60,000 feet), 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 mountainmaking 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. If the science of geology had had its origin on the Pacific side of America, some of its early generalizations would have been very different from those which gained credence in the lands bordering the North Atlantic. There is here exemplified, as might be expected in a region extending over 3000 miles from Alaska to southern Mexico, and measuring over 1000 miles 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 m 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