GEOLOGY. The geological history of North America, considered in a broad way, is not com plex. The oldest part of the continent., the first to be elevated above the sea, is the northeastern section, including the Adirondacks of New York and the Laurentian Highlands of Canada, and a region about the Great Lakes, together with a southward projection just east of the Blue Ridge in the Southern States. This is the Archaean area. From this, as a nucleus, the continent grew westward, as is indicated by the surface formations, which become successively more re cent. The eastern portions of the Appalachians are in great part composed of Silurian beds. The plateau forming the western part of the sys tem is Carboniferous, which formation also un derlies much of the Mississippi Valley. The great plains which form the eastward slope of the Cordilleran plateau are floored, in westward succession, by Triassic, Cretaceous, and Tertiary beds.
The mountains of the Cordilleran system are mainly of recent formation, and show strata of all ages, as they have been much disturbed by uplift, and the beds exposed by subsequent ero sion. Upon the mountains granitic rocks large ly predominate, as the stratified beds which form erly covered them have been eroded away, while in very many cases these stratified beds still re main on the flanks of the ranges. as hog-back ridges. The valleys are often partially filled with detritus from the mountains. In this re gion many great areas have been covered by out flows of lava, some of them in very recent times. The regions bordering the coasts of the Atlantic and the Gulf of Mexico are floored with Creta ceous and Tertiary deposits, indicating their comparatively recent uplift. There are no active
volcanoes in the United States proper or in Can ada. Within historical times eruptions have been reported on the coast of Alaska, and several peaks on that coast are still smoking. In 3Iex ieo, Central America, and the West Indies there are many active volcanoes. See Geology under UNITED ST tTES ; CANADA, etc.
Routh Ameriert.—The eastern highlands are of Arch can and Paleozoic formations, with a super imposed layer of sandstone. No subsequent sub mergence has occurred, and no folding has taken place since Paleozoic times, so that no recent marine deposits have been made, and the deep valleys are due to erosion rather than to irregu lar faulting, the rock layers lying horizontally. These eastern highlands are but the remains of a great mountain system which has been worn away to the existing condition in the filling up of the plains below, to which they have contrib uted their material. The western highlands (see ANDES ) , while of more recent origin than the eastern, are made up of ranges differing in geo logic age. Most of the great peaks of the Andes are of volcanic origin, and many of them are still active, or have been eruptive in recent and historic times. The lowlands east of the Andes are, so far as known, floored with Tertiary de posits, with broad hands of alluvium bordering the larger streams. See Geology under BRAZIL; ARGENTINA, etc.