PRECIOUS METAL INDUSTRY IN THE UNITED STATES. 65
bearing ores the following broad general features may be recognized. The mountain masses of Colorado are divided iu a general way into two north and south uplifts the Colorado or Front range, and the complex of rauges forming the Sawatch uplift with a third uplift, the San Juan group, at the south, whose greatest extension is east and west rather than north and south.
The two first named uplifts consist of a nucleus of Archean or ancient crystalline rocks surrounded by a varying fringe of upturned Paleozoic aim Mesozoic sediments, the whole cut through by dikes and intrusive sheets of eruptive rocks. The Paleozoic rocks are mostly limestones and quartzites; the Mesozoic rocks, sandstones and clay shales. Here the bulk of the silver-bearing ores are found in the Paleozoic limestones, while the crystalline rocks afford both gold and silver ores, and the Mesozoic rocks contain but few workable deposits. The preciousmetal deposits are invariably found in more or less intimate association with the eruptive rocks, and in a few cases gold-bearing ores are found within the latter and also in the Mesozoic shales immediately adjoining them.
The San Juan group is made of a similar series of rocks, but differently distributed, eruptive rocks forming the greater part of the surface exposures and the sedimentary and ancient crystalline rock masses being so broken up that the nucleal structure is no longer apparent. The greater pare of the precious-metal ores are found in the eruptive masses, being generally mixed ores carrying values in gold, silver, and other metals, but important deposits are found whose values are almost exclusively gold or silver. Precious-metal deposits are also found in the occasional exposures of Paleozoic limestones, and to a limited extent in the fringing Mesozoic sandstones.
In the Colorado or Front range the Paleozoic rocks which surround the Archean nucleus are mostly buried beneath the later Mesozoic sediments; consequently it is in the crystalline rocks and the associated eruptive masses that the principal precious-metal deposits have been found. The bulk of the product comes from the mining districts of Boulder, Clear Creek, and Gilpin counties. The ores of the former are rather unusual in that they consist largely of tellurides. Its product is relatively small, being less than half a million annually, and threefourths of its value is in gold. Clear Creek produces mixed base-metal ores, less than a third of whose precious-metal values is in gold. In Gilpin couuty, which is the oldest mining district in the State, the ores are mainly pyrites and of the precious-metal values 80 to 90 per cent, is gold. It has produced since 1860 about fifty-four millions of the precious metals. The combined annual product of these three districts has varied from four to a little over five millions of the two metals, having reached the larger amount at the beginning and again at the end of the decade. The proportion of either metal in the total product has also varied from year to year, as it comes from a very great number of min 92------5
