Mesozoic age. The uppermost of the latter series is the coal-bearing Laramie Cretaceous.
The precious metal product is derived from the older rocks, which are, in places, cut by dikes and intrusive bodies of later eruptives. Although, as is generally the case, it was the placer gold that first attracted miners to the region in 1875, these deposits are not very extensive, and the gold derived from them forms au inconsiderable portion of the gold product of the State, having averaged in round numbers about $50,000 in the early part of the decade and $30,000 per annum in the last few years, or less than one-twentieth of the total gold product.
It is deep mining in large and easily-worked bodies of extremely low-grade ore that has been the characteristic feature of the mining industry of the region, and has placed it on an unusually permanent basis. Pour large mines, now controlled by a single company, have in the last eight years contributed more than $20,000,000, or over fivesixths of the total of about $24,000,000 of gold produced in the region. Their deposits occur in the crystalline schists at the northern end of the range, in immense bodies sometimes 400 feet wide. The ore is a free milling gold ore, easily crushed, and practically free from other metallic combinations except a small amount of iron pyrite, so that, though its average yield is said to be from $2 to $4 per ton, owing to the large quantity treated, it can be worked at a profit, and the mines have paid about six million dollars in dividends to their owners. Although no ore bodies are inexhaustible and one mine of the group has already ceased producing, so that the product from these mines will' necessarily decrease in time, they have proved unusually permanent and the industry established by them has encouraged the building of railroads, which furnish fuel and other necessary facilities for cheap mining; hence it is reasonable to expect that the development of other bodies will gradually replace the falling oft' in their product. Four competing railroads now reach the region, where none existed at the commencement of the decade.
Gold-bearing ores also occur in the Potsdam or Cambrian sandstone. The most interesting, from a scientific point of view, are the cement or conglomerate ores at its base, which are considered to be old placers formed on the shores of the Cambrian ocean from the disintegration of the rocks of the original Huronian island, but since hardened, so that the ore is crushed and milled, and its product is classed with that of deep mines. Complicated base-metal ores, rich in gold, also occur in these sandstones, and at the end of the decade had begun to be mined on a considerable scale, the more complicated smelting or lixiviation processes necessary for the treatment of such ores having been rendered practicable- by the supply of cheap fuel brought in by the railroads.
Of the silver product of the State a small but regular amount is derived from the gold bullion of the gold belt mines, about 1£ per cent.
