68 GEOLOGY AlfD MINING INDUSTRY OF LEADVILLE. 



Weber Grits. — This formation, which, as its name imphes, consists mainly 

 of coarse sandstones passing into conglomerates, has an estimated aggregate 

 thickness of 2,500 feet, although neither its upper nor its lower limits can 

 in the nature of things be very sharply defined. 



The typical rock, which often forms massive beds of considerable thick- 

 ness and constitutes a prominent feature in the sections afforded by canons, 

 is a coarse white sandstone passing into a conglomerate, made up of well- 

 rounded grains and pebbles, mainly of white and sometimes of pinkish 

 quartz. In the coarser conglomerates feldspar can often be distinguished in 

 fragments, and this mineral is often disseminated in fine grains throughout 

 the sandstone, but fragments of recognizable Archean schists are not often 

 seen. It would seem, therefore, that these beds are mainly formed by the 

 abrasion of the coarser granites of the Archean. The sandstones often contain 

 a considerable admixture of brilliant white mica, and in some cases, besides 

 the mica, so large a quantity of carbonaceous material as to become quite 

 black. This carbonaceous material, which is insoluble in ether, alcohol, or 

 sulphide of carbon, is probably either graphite or anthracite. 



Next to the sandstones and conglomerates, the most important constitu- 

 ents of the formation are quartzose shales and mica schists, generally coarse- 

 grained and of a greenish hue. Their lamination is very regular and often 

 parallel to the bedding-planes, so that they often weather out in slabs or 

 flags of considerable size. The mica, which, as in the sandstones, is mostly 

 potash mica or muscovite, seems to form but a subordinate part of the rock 

 mass, but is generally very prominent in large brilliant flakes on the surfaces 

 of the laminae. Microscopical examination shows that in the sandstones 

 and schists feldspar is always present with the quartz, and in some cases the 

 three varieties, orthoclase, plagioclase, and microcline, can be distinguished. 

 It also shows that the muscovite is, in part at least, derived from the decom- 

 position of the feldspars; at the same time the uniform occurrence of large 

 brilliant flakes along the bedding-planes of the shaly material suggests the 

 possibility that these may have been directly derived from debris of the 

 Archean and have been deposited in this position by the action of water. 



At irregular intervals tlu-oughout the formation are found beds of fine 



