440 HISTORICAL GEOLOGY 



of this locality were folded, compressed, and raised into lofty 

 ridges which are now being carved by erosion into rugged 

 mountains. 



A little later, the Sierras, Rockies, and other western 

 ranges began a renewed epoch of growth ; this time not 

 chiefly through folding, as at the close of the Cretaceous 

 period, but by mere warping and faulting. The rise of the 

 Sierra range and its northward continuations consisted of an 

 arching of the surface; but locally, as along the east base 

 of the Sierra, the arch cracked (Fig. 457), or, in other words, 

 was faulted, and that side is now much steeper than the 

 slope toward the Pacific. There is good evidence that the 

 slow uplifting of the Sierra is still going on, for as recently 

 as 1872 a slip of nearly twenty-five feet occurred along this 

 fault plane. 



FIG. 457. Stereogram of a low fold broken on one side. 



Tertiary volcanoes. In this western region volcanic 

 eruptions continued, but with somewhat decreasing activity. 

 They have only very recently ceased, and it is in fact by 

 no means certain that the present is anything more than 

 a temporary period of quiet in that respect. Near the 

 middle of the Tertiary period, eruptions of lava from fissures 

 as well as from volcanic craters took place over a vast area 

 in the northwestern part of the United States, particularly in 

 Idaho, Washington, and Oregon. Flow after flow of liquid 

 lava welled up through cracks in the earth and poured out 

 over the surface, leveled up its inequalities, and finally pro- 

 duced a plateau more than a thousand feet in height and equal 

 in extent to several good-sized states (Figs. 25, 458). Simi- 

 lar eruptions have occurred occasionally in earlier periods, 

 but nothing quite like them has been observed in historic times. 



