7 i8 



ARTHUR C. TROWBRIDGE 



texture only, not the structure. The relations of coarse and fine 

 material can be seen to some extent on the surface of the plain. 



As has been shown above, the coarse and fine materials are more 

 or less separated on the surface of the plain. There are consider- 

 able stretches, usually the lower areas, where the surface material 

 is all fine. Such areas are interrupted by narrow belts of large 

 bowlders. If the structure of the whole plain were judged by its 



Fig. 8. — Fine material in the Sierra bajada seven miles from the mountains 



suriicial aspect, the material could be known to be roughly sorted 

 into many narrow radiating belts of coarse materials, and broader 

 belts of fine materials. Presumably these lines would not have the 

 same position horizontally for any considerable vertical section, as 

 the stream channels undoubtedly shifted and distributed often. 



So far as cuts in the bajada show, the materials consist of a 

 mixture of large blocks of granite, bowlders not so large, angular 

 fragments the size of cobbles, tiny angular bits of rock, sand, and 

 clay. Where any considerable vertical section is seen, these differ- 

 ent grades are sorted into indefinite lenses and pockets. There are 

 no definite layers of great extent. Divisions of material are no- 

 where seen to be continuous for as much as ioo ft. Some small 



