A considerable area, estimated at more than two million acres, may be made productive by irrigation, and the United States Government is undertaking the construction of an elaborate and extensive system of reservoirs for the impounding of stream and storm waters now running to waste. The irrigated lands of this region, when their products are accessible to markets, are very valuable. The river-bottom lands of New Mexico, and the old margins of Great Salt Lake in Utah are examples. They produce abundantly, and a single acre often yields as much as four or five acres in regions of plentiful rainfall. Not much of the crop of this region, the fruit and wool excepted, leaves the vicinity in which it is grown, on account of the expense of transportation. In the matter of the transportation of their commodities, the dwellers of the western highland are doubly handicapped. The building of railways is enormously expensive, and in a region of sparse population there is comparatively little local freight to be hauled. The difficulties of developing such a region from a commercial stand-point, therefore, are very great. Mining is the chief industry of this section, and silver, gold, and copper are its most important products. Since the discovery of precious metals in the United States, this region has produced gold and silver bullion to the value of about four billion dollars. This sum is about one-half the value of the railways of the country,[55] and from 1865 to 1880 a large part of the capital invested in railway building represents the gold and silver of these mines. In the last twenty years of the past century they produced an average of about one hundred and twenty-five million dollars per year, and this average is constantly increasing. Coal-measures extend along the eastern escarpment of the Rocky Mountains, and these are destined at no remote day to create a centre of steel and other manufactures. Several of the railways operate coal-mines in Colorado and Wyoming for the fuel required. A limited supply of steel is also made, the industry being protected by the great distance from the Eastern smelteries. [Illustration: GOLD MINING--CRIPPLE CREEK, COLORADO] _Denver_ is the chief active centre of finance of the mining industry in the western highlands, although many of the great enterprises derive the capital necessary to develop them from _New York_ and _San Francisco_. _Leadville_, _Cripple Creek_, _Butte_, _Helena_, and _Deadwood_ are regions of gold and silver production. _Virginia City_ is the operating centre of the famous Comstock mines. At _Anaconda_ is the chief copper-mine of this region. _Salt Lake City_ and _Ogden_ are the centre of the Mormon agricultural enterprises. _Santa Fe_, _Las Vegas_, and _Albuquerque_ are centres of agricultural interests and stock-growing.