THE COLUMBIA PLATEAUS, built up of great sheets of lava, lie between the Rocky Mountains and the Cascades, and cover the most of Washington, the north and east of Oregon, and the whole of Idaho south-west of the Rocky Mountains. Their altitude varies from 500 feet along the valley of the Columbia River to 4,000 feet, and over, in the more distant parts. The climate is not so extreme as it is farther east within the same parallels of latitude, the temperature being raised in winter by warm winds from the Pacific, and cooled in summer by the high elevation. Over the greater part of the region the mean winter temperature is seldom below 24° F., and in places it is as high as 36° F., while the summer mean varies from 60° F. to 70° F. The rainfall generally occurs during the cooler parts of the year, the summers being almost entirely dry, and the mean annual precipitation is, as a rule, under 15 inches, though in a few places it rises to 20 inches or even more. At the same time the capacity of the soil, like that of
the disintegrated Deccan trap, to absorb and retain moisture is so great that, although sage bush, mingled with bunch grass on which cattle graze, is the prevalent type of vegetation, large crops of wheat are grown over considerable areas on the same land every other year without the aid of irrigation. The most important of these wheat-growing districts are in the south-western part of Washington, where the Palouse valley with the surrounding country has long been noted for its productivity, in the northern parts of Oregon, where the most fertile districts lie just south of the Columbia River, and in the adjoining parts of Idaho. The Columbia Plateaus produce about 8 per cent. of the wheat crop of the United States.