GEOLOGY. 335 



much disturbed, and we frequently find them standing nearly 

 vertically. In the vicinity of San Francisco, the cuts through 

 the hills show great and numerous flexures. The aqueous 

 sandstone of California is generally unfit for either building or 

 road making. The stratification is thin ; it abounds in frac- 

 tures, does not wear well when exposed to the weather, and 

 under wheels is soon converted into mud. Some of it that 

 has been highly metamorphosed by heat, is excellent for build- 

 ing. 



261. Volcanic. Volcanic rocks occupy a large space 

 north of latitude 38. In remote ages, California was the.scene 

 of great volcanic activity in the northern half. No lavas or 

 volcanic peaks west of the summit of the Sierra Nevada have 

 been found south of the latitude of the Golden Gate on 37 

 48', while on the other side of that line they, are abundant. 

 Mount Diablo has the conical shape and solitary position of a 

 volcano, but its rocks are cretaceous. The numerous high 

 peaks of the Californian Alps the principal one reaching the 

 greatest elevation in the United States much as some of them 

 resemble volcanic cones at a distance, fail to show any signs 

 of volcanic action so far as they have been closely examined. 



Many of the fava beds of the Sierra Nevada are prominent 

 features of the landscape. They filled up the channels and 

 canons of the streams of the Pliocene or post-Pliocene age, 

 and being harder than the slates, the latter were washed away, 

 leaving those places which had been hollows standing like 

 steep mountains, rising 500 or 1,000 feet above the adjacent 

 country. The Tuolumne Table Mountain, 30 miles long and 

 half a mile wide, and the Oroville Table Mountain, nearly as 

 long, are the most remarkable examples of such geological 

 changes ; but many others might be found. Ridges covered 

 with beds of lava are common. 



There are immense beds of lava about Mount Shasta, and 

 appearances indicate that at least 10,000 feet of the elevation 

 of that peak are due to the matter ejected from its crater. 



