xciv GENERAL SUMMARY OF SCIENTIFIC AND 



and Cenozoic rocks of not less than 19,500 feet, marked by four 

 stratigraphical breaks. If to this we add 16,960 feet for the Car- 

 boniferous and Uinta groups, and 10,000 for the sandstones of the 

 Grand Canon, we shall have a total thickness of 40,460 feet from the 

 base of the Paleozoic series to the summit of the Tertiary in this 



region. 



WESTERN EOZOIC ROCKS. 



The study of the geology of this great western half of the Amer- 

 ican Paleozoic basin shows, as Gilbert remarks, that its rocks were 

 deposited upon a subsiding Paleozoic continent, over which we can 

 trace the advancing shore-line of the Paleozoic sea, which at length 

 covered all but some isolated and more elevated areas. The same 

 state of things obtained to the eastward also, where there is no 

 warrant whatever for the fomiliar notion, based on a singular mis- 

 conception, of a continent growing around a Laurentian nucleus. 

 The Paleozoic strata in this western region are every where clearly 

 distinguished by their uncrystallinc character from the crystalline 

 Eozoic rocks. In one case only, in New Mexico, quartzose schists, 

 holding hornblende and andalusite, are said to occur interstratified 

 with uncrystallinc strata canning Paleozoic fossils. The fact, how- 

 ever, that these strata are in parts vertical, and are traversed by 

 faults, and brought in contact with gneiss and crystalline schists of 

 admitted Eozoic age, suggests caution in fixing the horizon of these 

 supposed Paleozoic crystalline schists. Different observers have re- 

 ferred portions of these crystalline rocks to the Laurentian and the 

 Montalban series; but careful studies are yet wanting to establish 

 their true relations. 



METALLIFEROUS DEPOSITS. 



A great part of the deposits of the precious metals in this western 

 region are in veins in the Eozoic rocks, but it is well known that 

 they occur also in the Cambrian and the Carboniferous rocks; while 

 silver ores are found impregnating rocks of Cretaceous age, showing 

 that the transportation of the precious metals has been going on up 

 to a comparatively late period. 



ERUPTIVE ROCKS. 



Eruptive rocks, as is well known, are found penetrating and over- 

 lying the strata even of later tertiary age in the West. According 

 to Richthofeu, there was a natural and constant order in their ex- 

 travasation, the acidic rocks trachytes and rhyolites being al- 

 ways older, and the basic rocks basalts and dolerites newer, pen- 

 etrating and overlying the former. This would seem to be an in- 

 stance of hasty and imperfect generalization. The writer has long 

 since shown that some of tlie eruptive dolerites of Canada are of 

 pre-Paleozoic age ; and the observations of Le Conte and of James 



