Physiographical Conditions.— The great western chain of both continents, which would seem to be the chief formative base of both, is in fact very much the newest section in each case, though each is of independent origin, as shown by the energy of still remaining vol canic action in both; while the uplift of the eastern side has so long ceased that erosion has worn them down many thousands of feet, trenching immense valleys, and building up vast plains to the west by their detritus.
The Archean portion of North America, the first in order of appearance above the water, is the northeastern part: the elevation in which the Great Lakes and Hudson Bay are hollows, the Laurentian system of Canada, the Adiron dacks of northern New York, and a southern tongue east of the Blue Ridge. The line of forces thenceforth acted steadily to the west ward, the surface formations regularly grow ing more recent in that direction. This por tion is not merely the oldest of the western continent, but among the oldest on the globe, the ''New Worlds being new only from the standpoint of European history, not of geology or ethnology. This and the polar archipelagoes are composed of Pre-Cambrian or Palaeozoic rocks of extreme antiquity. The highlands of Labrador and those extending north and west are mainly granite and other archaic rocks. To the west stretches the vast plateau called by Suess the buckler° and known' to American geologists as the ((Canadian Shield.° By erosion this has been almost denuded of its upper Paleozoic strata, and the whole of Hudson Bay excavated to a slight depth on the surface of its eastern section. The eastern part of the Appalachian system is mostly Silurian and older rocks; its western plateau and most of the Mississippi Valley are Carboniferous; while as we go westward we encounter in suc cession Cretaceous and Tertiary formations.
The Rocky Mountain system shows the greatest activity of volcanic forces at its ends, in Alaska and Mexico. In the old portions of the United States and Canada there are no active volcanoes; and the strength of eruptive force, greatest in the Aleutian Islands, stead ily diminishes eastward and southward, oc casional eruptions occurring on the southwest ern coast, while Mount Wrangell is semi-erup tive only. Within the present boundaries of the United States, exclusive of Alaska, the only volcanic activity within historic times has been that manifested by Lassens Peak, California, during the years since 1914. In Mexico, Popo
catapetl and others indicate the beginning of the equatorial belt of volcanic forces exhibited in Central America and the Antilles. But all the Cordilleran system is relatively of recent eleva tion, though old enough for heavy erosions to have taken place, exposing strata of every age as they were tilted up, creating some valleys and filling up others. In the region from California to Puget Sound the surface over many thousand square miles is lava, the valley of the Snake and Columbia for long distances being cut through lava beds, and fields of black scoriae forming a peculiar feature in the northern Pacific States and Pacific Canada. To the south of the Appalachian system, along the Atlantic and the Gulf, the flooring is Cretaceous and Tertiary, therefore of recent uplift.
Modifying the erosive action on these primi tive elevations there has operated a vast ice cap, the so-called Laurentian glacier, which at an un certain but relatively recent period, ending probably from 50,000 to 100,000 years ago, covered all North America from the polar regions down to Philadelphia and the Ohio and as far west as the Missouri, leveling hills and hollows, creating soils, excavating lake beds, changing the courses of streams and the outlets of gigantic lakes, cutting out and blocking up fiords and harbors, and de positing enormous masses of rock and gravel moraines. To this, among other things, is due the creation of New York harbor and Niagara Falls, and the turning of the Great Lakes through the rocky Saint Lawrence valley instead of the Mohawk and Hudson. This ice cap has by no means wholly disappeared yet: the immense glaciers on the northwest coast and in the Rocky Mountain heights, some of them hundreds of square miles in extent, are remnants of the one great glacier of ancient times which still covers almost entirely the turtle-back conformation of Greenland.
In South America the eastern highlands are also of enormous antiquity, as shown by their archaic composition, with a sandstone cap not since submerged, their horizontal layers, deep erosions and detritus plains indicating no up lift since the earliest times. The Andes (q.v.) are quite recent, and full of volcanoes still or but recently active, but they are not all of a sin gle age, however, and show successive uplifts. The plains between have Tertiary bases under their alluvial surface.