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6 GUIDEBOOK OF THE WESTERN UNITED STATES. 
part of the United States. The history and the phenomena of this 
glaciation are considered in detail at several places in this book. 
The general features of the country west of the Mississippi are 
represented on Plate I. When, after crossing the Great Plains, the 
traveler reaches the foothills of the Rocky Mountains he will have 
attained a height of 4,000 feet above the sea, a height reached by few 
peaks in the Eastern States outside of the Adirondacks and the White 
Mountains. The Rocky Mountains form a great, irregular, rough- 
hewn “backbone” for the continent. They comprise many groups 
of ranges, in which some peaks in Montana and Idaho reach a height 
of 12,000 feet above sea level and some in Colorado rise more than 
14,000 feet. 
The western mountains, like the eastern, are the worn remnants of 
upward folds or crumplés or of upheaved blocks of the fractured 
earth crust, but, unlike the eastern mountains, which are geologically 
old, the western mountains are geologically very young. They are 
therefore higher, for since they were uplifted there has not been time 
for ice, rain, heat, frost, and wind to wear them down to lower levels. 
West of the Rocky Mountains lies a broad interior basin, in the 
northern part of which, with its inclosing mountains, there is sufficient 
rain and snow to maintain the flow of the great Columbia River; 
but in the southern part, in what is known as the Great Basin, the 
mountain streams find no outlets to the sea, their waters, so precious 
for irrigation, being soon lost in the thirsty lowlands, and the feeble 
or intermittent rivers of the valleys carry their waters down to be 
evaporated in alkali marshes or on saline deserts. 
The part of the Columbia River basin or plateau that is traversed 
by the Northern Pacific Railway is made up of lava flows, among 
the greatest in the world, which in comparatively recent geologic 
time spread like a fiery flood over hundreds of thousands of square 
miles; and a wide expanse of hard, dark volcanic rocks, whose sur- 
face is here and there cut deeply by streams, shows the enormous 
extent and volume of these eruptions. The part of this old lava 
plain that is crossed by Columbia River is the most arid region tray- 
ersed by this route. The precipitation in this region is sometimes 
not more than 6 inches annually, but despite the small rainfall 
the uplands have become the great wheat-raising country of the 
Northwest. 
The last great natural feature to be crossed by the traveler is the 
Cascade Range, which separates the interior basin from the region 
of Puget Sound. This range is a broad upland that stands from 6,000 
to 8,000 feet above the sea, and here the evidences of voleanic activity 
continue to be conspicuous. On the flanks of the range rise the snow- 
covered peaks of Mount Rainier, Mount Adams, and other cones, 
which were once active volcanoes, pouring forth streams of lava and 
