Geology.— Throughout the Paleozoic era, the region of the Rocky Mountains was low and featureless, and under the sea a great part of the time but with several local emergencies. Toward the close of the era, in the Permian epoch, it was probably a desert, and at this time were laid down the red sandstones and gyp sum beds that form so striking a feature of the eastern ranges. Land conditions obtained through Triassic and most of Jurassic time, but the region was not mountainous. 'Again in the Cretaceous most of the region sank beneath the sea for a short time. During all its history thus far the most important fact was the ac cumulation over all the region of the great series of sedimentary rocks that make the bulk of the mountains. During the late Cretaceous the region again became land, low and swampy, with the accumulation of coal beds. At the close of the Cretaceous there began a period of folding, which compressed and crumpled the rocks and arched them up into a great series of folds that constituted the first appearance of the Rocky Mountains. In the northern Rockies the compression was so great that the rocks broke and passed into enormous overthrust faults (q.v.) along which the beds slipped many miles. Following the uplift erosion set in and has been continuously. carving the region ever since. During Tertiary times the area was the site of very extensive volcanic activity that poured out the great lava flows of the stone Park and other localities. There have been one. pr two later periods of slight uplift and faulting, but no great folding since the close of the Cretaceous. The general trend of the folds and hence of the mountain ranges is nearly north and south, though a few lie nearly east and west. Erosion has planed off many of the folds, so as to expose cores oi older granite, from which sedimentary beds dip out ward in all directions. In the southwestern part of the system, faults are much more important than at the north, and great fault scarps are common.
Minerals and Mining.— The Rocky Moun tain section is the great treasure region of North America. Almost all the precious and useful minerals are found here in abundance. Gold had been discovered and used by the abo rigines, and deserted mines have been found where mining had been carried on in a crude way as early as 1680. Gold exists in nearly all the ranges of the system. Gravel containing coarse gold is found from 20 to 60 and even 100 feet below the surface. Abandoned gold fields were worked over with great profit during the last decade of the 19th century, by using improved methods. There are rich deposits of silver, copper, iron, salt, coal, lead and in the southwestern part of the United States, petro leum and gas. The placer gold regions in Alaska, and the gold fields of the same territory, have been found thus far of great extent and value. The value of the fine building stone, the fire-clay, gypsum, all kinds of cement ma terial, are rarely taken into consideration be cause overshadowed by the value of the gold, silver and copper. The gypsum beds in the northwestern part of Texas and along the foot hills of the ranges in southern Colorado are of great extent and value. For further details on minerals and see articles on the sepa rate States of the United States in the Rocky Mountain region, also CANADA and MExico.
Climate.—The west slope of the Rocky Mountains, or all that portion which faces the Pacific Ocean, has a milder climate than the central or eastern part of North America in the same latitude. The warm winds from the Pacific Ocean bring heat and moisture. The
rainfall is copious on the western side of the mountains. Like the 'mountain regions on all parts of the globe, the temperature lowers with increase of altitude, and the winds which here blow over the cold mountain tops and across the plateau region to the eastern slope carry with them neither heat nor moisture. In the southeastern part of this vast region where the mountains become low ranges or foot-hills, in the United States, west of the ranges that parallel the coast, the climate is hot in summer and mild in winter; in the Great Basin region and on the eastern slope of the mountains, extending out on the Great Plains, the climate is marked by aridity. See UNITED Smves.
Flora.—The great aridity of the region east of the western or coast barrier prevents the growth of vegetation. The surface is exposed to erosive action Which is especially rapid at such great elevations, and the denudation be comes more complete as the sand and small dis integrated fragments are swept away by the winds, thus giving no opportunity for the accu mulation of soil. A large portion of the cen tral and eastern mountain region and the west ern part of the Great Plains require irrigation In 'order to have vegetation; this is so even in the parts where these is abundant fertile soil, The arid portion of the United States, exclu sive of Alaska, is about two-fifths of the whole area. The redemption of the arid' lands is one of the nation's problems. Less than 10 per cent of the mountain region is forest4-clad ; but a larger proportion is covered with verdure,' in cluding the portions covered with dwarfed herb age, sage brush and stunted cedars and pines, most of which is useful for fuel but not tor building purposes. Crops that mature in a short season and pasturage are found on the foot-hills and valleys, especially along the streams. In some places where a little moisture is carried over the passes, below the snow line, the mountains are covered• with the dark ever green growth of hemlock, spruce, balsam, fir and talLpines, and still further down are inter spersed among them the birch, beech, cherry and various other trees, while along the streams are found groves of cottonwood and willow, Over large districts, however, the forest growth is often exceedingly sparse, and even the grass that covers the plains, and upon which travelers depend for the sustenance for their animals, is parched and disappears in the long droughts, to which these regions so remote from the sea are subject In the sandy regions along the North Fork of the Platte River above Laramie River an extraordinary growth of artemisias and other odoriferous plants .is found. They abound in the river bottoms and on the growing to the height of two and three feet in tough, twisted, wiry clumps. A multitude of flowering plants abound in this region, among which prevail several of helianthus sun flower), and in the month of September, when they are mostly in bloom, the whole country' resembles a vast' garden. On the western slope may be found some of the largest trees in the world, many of. them over 2,000 rat% old. The vast forests of the Northwest yield enormous quantities of timber. Not only is there a great variety and amount of plant life, but the vege tation attains a remarkable size. The shrub of the Atlantic region becomes a tall tree on the Pacific Slope. Like the mountains themselves, the vegetation which covers the western slope is large, massive, dense and in every way seems patterned on a gigantic scale.