194 CALIFORNIA ACADEMY OF SCIENCES. 



which in places black, evidently recent', volcanic rocks rest. 

 The range bordering the Salt Lake is probably entirely 

 volcanic. 



The desert between the foot of the escarpment and the 

 range just mentioned above, is 20 miles wide and slopes 

 gently from both sides to a maximum depression in the 

 middle. The elevation of this desert at the foot of the 

 Sierra is probably about l,Ol)0 feet, and the center lies pre- 

 sumably considerably below the level of the sea. The 

 same depression connects northward with the New River 

 Basin, and at its northern end Prof. Blake in the Pacific Bail- 

 road Reports estimates it to be 500 feet below the surface of 

 the sea. Southward it connects with the delta of the Col- 

 orado River by a gap in the desert range, a little south of 

 east from Campo Nacional. Near that point it must, of 

 course, be higher, or the river would flood the desert. 

 South of this gap the desert ranges continue, and the aver- 

 age elevation of the desert itself increases. 



CONCLUSIONS. 



Prof. Blake's section from San Diego to the desert, re- 

 ferred to above (see ante p. 177), presents certain similarities 

 and certain differences when compared with the one de- 

 scribed in these notes. Both sections have a gentle west 

 and a steep east slope, although this peculiarity is much 

 more accentuated in the southern profile. 



In both, the principal mass is composed of an enormous 

 granitic plateau with minor areas of highly metamorphosed 

 and compressed slates. In the former these are main- 

 ly confined to the eastern, while in the latter they are 

 intercalated in the western slope. The large masses of 

 mesozoic eruptives at the Pacific Coast are not represented 

 in the northern section. The granite composing the main 

 area appears everywhere to be identical — a white horn- 

 blende granitite similar to that of the Sierra Nevada of Cal- 

 ifornia. 



