three years that have elapsed since their discovery it has produced about one hundred and five millions of gold and three hundred millions of silver. In contrast to that of California, the development of its mining industry has been comparatively slow, and of the above-mentioned amounts nearly one-half the gold and five-sixths of the silver has been produced in the last thirteen years.
As in other regions, it was the discovery of placer gold that first attracted the miner in 1859-'60 to what was then a comparatively unknown region. As to what was the annual product of these early years estimates vary widely and nothing is certainly known. The vein mining, which followed the exhaustion of the richer placers, was conducted under a disadvantage, for most of the ores contained a great many other metallic minerals besides gold and silver hence constituting what is called base metal ores and could not be reduced by the simpler process of amalgamation.
It is in the older crystalline and eruptive rocks which were the first to be prospected that the gold-bearing ores are mainly found, whereas the great bonanzas of silver-bearing ores have been found in Paleozoic limestones, .and it was not until the discovery of the latter ores at Leadville in 1878-'79 that their value was recognized and prospectors paid more attention to their surface indications, which before had been considered valueless.
Smelting plants were necessary for the reduction of these ores, and it is their great increase that has been the most important factor in the rapid increase in the mining industry of Colorado since 1880. To those established in the immediate vicinity of the mines have been added great central plants at Denver and Pueblo as well as in Eastern cities, which, owing to their proximity to coal fields and to their ability to receive ores by rail from every part of the State, and even from other States, can work more cheaply and to greater advantage. The cost of smelting has thus been reduced as much as 50 per cent, during the decade. This has reacted favorably on the development of mines, since by the ready market thus afforded for their product a great many mines have been opened and worked the ores of which could not otherwise have been reduced at a profit. The effect of this industrial development has been most marked in the silver product, since the greater part of the silver ores must be reduced by smelting, whereas gold ores are more generally treated by amalgamation. The effect upon the production of gold has also been beneficial, since there are ores carrying gold, such as the tellnride ores of Boulder county and the concentrates from the tailings of gold mills, from which the gold can only be extracted at a profit by mixing with other ores in the larger smelting works. Of late years, moreover, numerous wet processes for the extraction of gold from complicated ores have been successfully introduced in various parts of the State.
In considering the geological distribution of the precious-metal
