| OCR Text |
Show 14 BASIS OP AMERICAN HISTORY [ 1500 and provincial boundaries, farther south the Roanoke, Yadkin, Catawba, and Savannah may be mentioned, while the southern plain is drained into the Gulf of Mexico by the Appalachicola, the Alabama, and other small streams besides the lower waters of the great Mississippi. The true continental watershed runs irregularly through the Rocky Mountains, and west of that barrier the drainage is to the Pacific, except for a few enclosed basinsT The chief western streams are the Colorado to the Gulf of California, the Columbia and the Fraser to the ocean, and the Yukon into Bering Sea. Geologically speaking, the record of North America is simple. The oldest part of the continent is doubtless the Laurentian plateau in eastern Canada, with its extension south represented by the Adiron-dacks of New York, and a considerable area about the Great Lakes. From this centre the emergence seems to have been progressively westward. The Laurentian uplift was evidently in the earliest geological periods, for the structure of the rocks and the character of the deposits argue a formation deep beneath the earth's crust, with a subsequent upheaval and gradual denudation, requiring ages to effect. The Cordilleras of the west are, on the contrary, comparatively recent and exhibit the results of late volcanic action. In Mexico, active volcanoes still exist, and in Alaska they are not entirely extinct. |
