552 GEOLOGY. 



very different from those which prevailed farther east. In the region 

 of productive coal, marine conditions seem to have recurred more 

 frequently and to have been of longer duration in the western coal- 

 fields than in the eastern. These relations suggest that now and 

 again the marine conditions of the west extended to the east, as the 

 result of subsidence in that direction, though it does not appear that 

 the sea-water at any time became deep within the regions where the 

 coal occurs. On the other hand, it now appears that the conditions 

 of the east sometimes prevailed, at least locally, in the west, for car- 

 bonaceous beds, and even beds of coal, are known in the system in Ari- 

 zona 1 and perhaps in New Mexico. 2 In some parts of the west, the 

 Carboniferous system includes formations which closely resemble 

 the " Red Beds " of the Permian. This is true, for example, in some 

 parts of Nevada, 3 where the separation of the Fennsylvanian system 

 from the Permian is not very distinct, or has not been carefully worked 

 out. 



The Carboniferous (Pennsylvanian) system of the west includes 

 all sorts of sedimentary rocks, among which are considerable thick- 

 nesses of limestone. They are exposed at many points (Fig. 240) 

 and their existence over wide areas where they are now covered by 

 later deposits is certain. The Carboniferous of the west is, however, not 

 continuous. Numerous islands of older rock probably maintained 

 themselves throughout the period, and a large area of land existed 

 throughout the Paleozoic era in western Nevada (west of long. 117°) 

 and had an unknown extension north and south. 



In the Carboniferous system of some parts of Colorado (Elk 

 Mountains and Sangre de Cristo range) and New Mexico, there is a 

 notable unconformity, with great accumulations of conglomerate, 4 

 3,000 to 6000 feet thick, at the base of the upper (probably Penn- 

 sylvanian) system. Some of the bowlders of the conglomerate are 

 said to be 50 feet in diameter. The unconformity seems not to be 

 universal, and the conglomerate is, so far as known, rather local. Both 

 phenomena have been attributed to notable orogenic movements, 



i Blake, Am. Geol., Vol. XXVII, 1901, p. 161; also Durable, Am. Geol., Vol. XXX, 

 p. 270. 



2 Southwest of Las Vegas, unpublished evidence. 



3 Spurr, Bull. 208, U. S. Geol. Surv., pp. 166-173. 



4 Emmons, Orogenic Movements in the Rocky mountains, Bull. Geol. Soc. Am., 

 Vol. I. 



