GEOLOGY. 461 



geology of the Eureka district in ^Nevada, with the help of Walcott in 

 paleontology and Iddings in lithology. This district was selected both 

 as a typical region for the study of tbe geology of the Great Basin, and 

 also as one of economic importance from its mines of precious metals. 

 From the great plateau, here about 0,000 feet above sea-level, the Eu- 

 reka Mountains rise from 2,500 to 4,000 feet, and form an almost de- 

 tached mass of Paleozoic strata, with eruptive rocks of Paleozoic and 

 more recent dates. The Paleozoic sediments of the region have been 

 broken up by an intricate system of faults, with flexures, into several 

 great masses or blocks, from a comparative study of which it has been 

 possible to reconstruct the geological succession, and with the aid of 

 paleontology to give a more complete view than has yet been obtained 

 of the Paleozoic series of the Great Basin. The strati graphical col- 

 umn, from the lowest exposed beds of the Cambrian to the summit of 

 the coal measures, has a thickness of not less than 30,000 feet, of which 

 over 11,000 feet are limestones or dolomites, and 5,000 feet quartzite, 

 and presents but a single break, which, marked by unconformity, ap- 

 pear in the midst of the second fauna. The name of Silurian is 

 given by the author to the rocks holding the second and third faunas 

 of Barrande, that of Cambrian being reserved for the first, but in 

 the great conformable sequence here displayed the transitions between 

 these three faunas are marked by such gradations that the dividing 

 lines adopted for these divisions are confessedly arbitrary. 



Of the 7,700 feet assigned to the Cambrian, the lowest member, the 

 base of which is not displayed, is the Prospect Hill quartzite, 1,50 > feet 

 in thickness, to which succeeds the great mass of more or less magnesian 

 limestone of 3,000 feet, known by the same local name. Between these 

 two occur the first fossiliferous beds, known as the Olenellus shales, 

 which have afforded a fauna closely related to that of the slates of 

 Georgia, in Vermont, a portion of the old Hudson-Biver group, subse- 

 quently called Lower Potsdam by Billings. In the great mass of the 

 Prospect Mountain limestone and its overlying Secret Canon shales, with 

 more or less limestone (2,000 feet), followed by the Hamburg limestone 

 (1,200 feet), and the Hamburg shales with chert nodules and layers (500 

 feet), which is regarded as the summit of the Cambrian, we have at va- 

 rious horizons an abundant fauna, which is closely related to that of 

 the Potsdam of the Upper Mississippi. We have thus in conformable 

 succession in this region the divisions hitherto called Lower and Upper 

 Potsdam, the two ranging through more than 6,000 feet of strata. To 

 the Hamburg shales succeed the Pogonip group, consisting of 2,700 feet 

 of limestones, with some argillaceous and arenaceous beds, overlaid 

 by the Eureka quartzite, compact, vitreous, and 500 feet thick, without 

 fossils. 



The forms of the first fauna pass upwards from the Hamburg shales 

 some distance into the Pogonip group, which higher up contains an 

 abundant fauna, compared with that of the Chazy, with some forms 



