582 HISTORICAL GEOLOGY 



composed of 8000 feet of sediments, largely belonging to the Upper 

 Miocene, as well as an equal amount from the earlier stages. Volcanoes 

 had practically ceased to be active over a large portion of the territory, 

 but were probably still in eruption in some localities. The Miocene 

 deposits on the Pacific coast are much folded ; and some are even 

 overturned, being in marked contrast in this particular to those of the 

 Atlantic and Gulf coasts, which are nearly in the position they had 

 when first laid down. 



In addition to the marine sediments just discussed, continental 

 deposits, consisting of sands and clays with some iron and coal, were 

 being laid down during the Lower Miocene in the northern part of 

 the Great Valley. From the western flanks of the Sierra Nevadas 

 auriferous gravels were carried down by the streams and dropped in 

 their beds during portions of the period, producing the " deep au- 

 riferous gravels " (Fig. 359 B, p. 374) and later the " bench gravels," 

 some of which, as now, were buried beneath streams of lava and beds 

 of tuff. 



Mountain Building. — Before the close of the epoch the upheaval 

 of the Coast Ranges of California and Oregon and the Cascades of 

 Washington occurred ; the fault along the east of the Sierra Nevadas 

 was made ; the growth of the present Sierra Nevadas was begun and, 

 as will be seen later, many of the great mountain ranges of the world 

 were elevated. During this epoch, too, the plateaus of Utah and Ari- 

 zona were raised so as to permit the Colorado River to begin the 

 excavation of its great canyon. The rugged scenery so characteristic 

 of the west is the result of elevation which, for the most part, began 

 at this time. 



Basis for Separation into Periods. — In the discussion of eras and periods attention 

 has frequently been called to. the fact that they were brought to a close by deforma- 

 tions, some great and some small, which produced mountain ranges, or raised or 

 lowered large areas of the earth's surface. We have just seen, however, that one 

 of the great times of mountain building occurred, not at the end of an era, nor the 

 close of a period, but in the midst of an epoch. It should also be remembered that 

 climatic and other changes thus produced had little effect on the contemporary life of 

 the time. In other words, the separation of the history of the earth into chapters 

 should be based, not upon the unconformities, however great, but upon the changes 

 which the life has experienced. Fortunately, as should be expected, because of the 

 effect of the physical conditions upon animals and plants, the sediments laid down 

 during eras and periods are usually to be separated, not only by the rather sudden 

 extinction of many species and the appearance of new ones, but by unconformities as 

 well. The problem is not, however, a simple one. When, for example, a continent 

 1 n isolated for long ages, the animals and plants living on it may be largely 



