36 CALIFORNIA ACADEMY OF SCIENCES [Proc. 4th Ser. 



The San Jose Range is the dominating feature of the district 

 on the south. It rises to a maximum elevation above 4000 

 feet. The numerous spurs and ridges descending from it have 

 no definite orientation, but strike obhquely or at right angles 

 from the main divide, which trends in a northwest and south- 

 east direction. 



While the general area of the San Juan district is but little 

 less than 2000 feet above sea level it has relatively low relief 

 when compared with the more rugged topography of the 

 neighboring range. The hills are well rounded and the slopes 

 gentle. The low ridges and spurs have a general northwest 

 and southeast direction, conforming to the larger topographic 

 features. Recent stream erosion has cut deeply into the softer 

 Tertiary formations, and all of the streams, and even the larger 

 gulches and ravines, have fairly well-developed flood plains 

 within the area of these formations. Within the area of older 

 rocks the ravines and gulches are steeper and narrower, and 

 with hardly any flood plains developed. 



Geology 



Basement Rocks. — The basement on which the Neocene and 

 later sediments rest in the area covered by the mapping is 

 almost entirely of granite. There are a few scattered patches 

 of limestone in this area but they are mostly small and rela- 

 tively unimportant. They are probably remnants of some 

 paleozoic formation into which the granite has been intruded. 

 A larger body of such limestone lies outside of the area covered 

 by actual mapping, toward the north. It makes up a large 

 part of the floor of Cholame Valley to the north and east of 

 Shandon. 



The boundaries of the basement rocks have not been mapped 

 in detail. They compose the general areas of the Coast Ranges 

 which represent the insular land masses of the early Neocene 

 in the Temblor Basin. The San Jose Range lying immediately 

 to the south of the San Juan district is composed almost en- 

 tirely of granite. Along the eastern flank of the granite on the 

 upper tributaries of the San Juan there are some sandstones 

 and shales that are probably Cretaceous, and both Cretaceous 

 and Eocene rocks are found in the Temblor Range and are 

 indicated on the areal map of the McKittrick-Sunset district. 



