ward to form the earth's first hills. The crust was then so thin and easily broken that only low hills appeared but after it became thicker it was not so easily rent and mountain ranges were born with an accompanying roar of tremendous volcanoes. Geologists, studying the ages of the different ranges in the United States, have concluded that the first to appear were the Appalachian mountains; later the Rocky mountains in the west, and still later the Coast ranges. Mountain ranges are invariably found along lines of very thick sediments. The same strata which along the Appalachian range are 40,000 feet thick, thin out, when traced westward, to only 4000 feet at the Mississippi River.[1] The high tablelands in portions of Idaho, Oregon and Washington were formed by great streams of lava which poured through many of the fissures of the coast ranges and buried the whole country above the hill tops. This lava cooled into thick massive layers of solid rock—an interesting sight to this very day.
Although mountain ranges appear amid violence and their growth, in a geologic sense, is rapid, plains and plateaus are illustrations of sea-bottoms which have been covered with sediment carried down from the mountains,—mainly pebbles, sand and clay,—pressed and cemented together under the water and then gently raised to the surface. Sometimes the sea-bottom is exposed as a vast plain; again the strata are upheaved, bent or broken, forming faults and ridges. Many of the elevated plains and plateaus are later sculptured into hills by rains, winds, frosts and rivers. The hills are then gently torn down, the valleys filled up and the sediment again deposited by the rivers upon the bed of the ocean.
A few million years passed by and these same deposits may again be raised and become dry land. The great stretches of land be-
- ↑ Le Conte.
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