338 SCIENTIFIC RECORD FOR 1882. 



posed sheets, wliicli were spread out between deep-lying sedimentary 

 strata before tbe folding and faulting of the latter. A layer of white 

 quartziferous i)orphyry overlies the blue limestone in the lower part of 

 the Carboniferous, and varies from 1,000 to 100 feet in thickness, or thins 

 out altogether. Other sheets of eruptive rock are found in the quartz- 

 ite below and at various other horizons, in parts of the region as high 

 as the Jurassic. The ores of the district, principally argentiferous 

 galena, and the products of its alteration, are found chiefly in the blue 

 limestone beneath the great porphyry sheet, but also in the white dol- 

 omite below, and even in the quartzite. They occur, as is usually the 

 case in deposits of this nature, in irregular cavities in the limestone. Em- 

 mons is disposed to believe the source of the metals to be in the erui)tive 

 rocks, from which they have been dissolved by infiltrating waters and 

 deposited among the sedimentary rocks. But it is to be remembered 

 that we find similar ore-deposits in limestones where eruptive rocks are 

 absent, as in parts of Nevada, for example, and in the Mississippi Val- 

 ley. The deposition of the ores at Leadville is, however, shown to have 

 been posterior to the intrusion of the igneous rocks and anterior to the 

 faulting of the strata. 



GRAND CANON OF THE COLORADO. 



Dutton, in his studies of the Grand Canon district, just published by 

 the United States Geological Survey, has given important details with 

 regard to its geological history, which throw light on the differences ob- 

 served in the thickness of the Paleozoic rocks in different jiarts of the 

 great western region. He shows that while Cambrian, Siluriau, and 

 Devonian beds are frequently met with, the great Carboniferous series, 

 for the most part, rests directly in this region on the Eozoic. Where 

 the older Paleozoic strata underlie the Carboniferous, they are uncon- 

 formable and have been subject to erosion. This is the case in Nevada, 

 Utah, and in central and western Arizona. Beginning at the base of 

 the Carboniferous, there is throughout this region apparently a contin- 

 uous and conformable series of sediments to the top of the Cretaceous. 

 The Grand Caiion of the Colorado, some 5,000 or 6,000 feet in depth, is 

 excavated from 4,000 to 4,500 feet in the Carboniferous, the remainder 

 being in Eozoic and in some parts in lower Paleozoic strata. The 

 Carboniferous here has at its base about 1,800 feet of limestone, followed 

 by shales and sandstone, and terminated by about 700 feet of limestone. 

 Above this come Permian, Triassic, Jurassic, and Cretaceous rocks, suc- 

 ceeded by the Eocene, which forms the High Plateau region of southern 

 Utah, the interval from the top of the Eocene to the Carboniferous be- 

 ing there from 4,000 to 5,000 feet. Eastward, towards the Uinta Mount- 

 ains, the Eocene itself, however, attains a greater thickness, making 

 there the whole volume of deposits above the Carboniferous not less 

 than 10,000 feet. Dutton concludes that while the lower limestones of 

 the Carboniferous were formed in somewhat deej) water, the whole 



