Fifty-five per cent, or more than one half of the total land surface of the earth, receives an annual precipitation of less than 20 inches, and must be reclaimed, if at all, by dry-farming. At least 10 per cent more receives from 20 to 30 inches under conditions that make dry-farming methods necessary. A total of about 65 per cent of the earth's land surface is, therefore, directly interested in dry-farming. With the future perfected development of irrigation systems and practices, not more than 10 per cent will be reclaimed by irrigation. Dry-farming is truly a problem to challenge the attention of the race. CHAPTER IV DRY-FARM AREAS.--GENERAL CLIMATIC FEATURES The dry-farm territory of the United States stretches from the Pacific seaboard to the 96th parallel of longitude, and from the Canadian to the Mexican boundary, making a total area of nearly 1,800,000 square miles. This immense territory is far from being a vast level plain. On the extreme east is the Great Plains region of the Mississippi Valley which is a comparatively uniform country of rolling hills, but no mountains. At a point about one third of the whole distance westward the whole land is lifted skyward by the Rocky Mountains, which cross the country from south to northwest. Here are innumerable peaks, canons, high table-lands, roaring torrents, and quiet mountain valleys. West of the Rockies is the great depression known as the Great Basin, which has no outlet to the ocean. It is essentially a gigantic level lake floor traversed in many directions by mountain ranges that are offshoots from the backbone of the Rockies. South of the Great Basin are the high plateaus, into which many great chasms are cut, the best known and largest of which is the great Canon of the Colorado. North and east of the Great Basin is the Columbia River Basin characterized by basaltic rolling plains and broken mountain country. To the west, the floor of the Great Basin is lifted up into the region of eternal snow by the Sierra Nevada Mountains, which north of Nevada are known as the Cascades. On the west, the Sierra Nevadas slope gently, through intervening valleys and minor mountain ranges, into the Pacific Ocean. It would be difficult to imagine a more diversified topography than is possessed by the dry-farm territory of the United States. Uniform climatic conditions are not to be expected over such a broken country. The chief determining factors of climate--latitude, relative distribution of land and water, elevation, prevailing winds--swing between such large extremes that of necessity the climatic conditions of different sections are widely divergent. Dry-farming is so intimately related to climate that the typical climatic variations must be pointed out.