PHYSICAL DIVISIONS AT EUEEKA. 185 



culties has been to connect the Cambrian and Lower Silurian rocks below 

 the Eureka quartzite with the Upper Silurian and Devonian above it. 



Again, from a physical point of view there are obvious reasons for 

 linking together in one group the enormous development of limestones 

 lying between the Eureka quartzite and the Diamond Peak quartzite. So 

 imperceptible is the transition in physical characters between the Lone 

 Mountain beds of the Silurian and the Nevada limestone of the Devonian 

 that no line can be sharply drawn, and while the fauna slowly undergoes 

 change there are hundreds of feet of sediments that might as well be placed 

 in one as the other of the two formations. Atrypa reticularis and other species 

 found near the base of the Devonian have at other localities been obtained, 

 associated with species regarded as belonging to the Silurian. 



With the coining in of the Diamond Peak epoch another great change 

 takes place in the physical conditions, and with it a marked faunal break, 

 which brings in the Carboniferous period. From the Diamond Peak quartz- 

 ite to the summit of the Paleozoic rocks the beds form one natural group, 

 no matter from what point of view they may be considered. The litholog- 

 ical distinctions hold good over wide areas. 



From the standpoint of physical geology the record shows three grand 

 divisions: first, one with the Prospect Mountain quartzite at the base, fol- 

 owed by a series of limestones and shales; second, beginning with the 

 Eureka quartzite, followed by another series of limestones to the top of 

 the Devonian; third, a great quartzite belt, followed in turn by a lime- 

 stone, a conglomerate, and a second limestone. 



LOWER PALEOZOIC IN ADJOINING REGIONS. 



After the completion of the field work, upon revisiting several of those 

 ranges in the Great Basin where the descriptions given of the lower Paleo- 

 zoic sections differed essentially from the sections exposed at Eureka, it 

 was found that, as regards sequence of beds, they stood singularly in accord 

 and could easily be correlated with those of the latter locality. A knowl- 

 edge of the sedimentary beds at Eureka serves to unravel in neighboring 

 ranges several knotty problems previously not clearly understood, and to 

 show that similar physical conditions existed over a wide area of ocean 



