44 



CALIFORNIA ACADEMY OF SCIENCES 



[Proc. 4th Ser. 



Monterey Shale. — As in many other localities in the Coast 

 Ranges of California, the Temblor group is overlaid along the 

 San Juan River by a series of light colored organic shales with 

 apparent, though probably not actual, conformity. The strati- 

 graphic position and lithologic character of these beds make 

 them conspicuous, and their strong lithologic contrast with the 

 terriginous beds which they overlie makes the mapping of their 

 contact comparatively simple. As there is no evidence, faunal 

 or other, that they form a part of the underlying group, they 

 have been referred to as the Monterey division of the Neocene. 

 Their composition varies considerably within this district, 

 though the predominating type of rock appears to be a mixture 

 of siliceous, organic and bituminous shale, with terriginous 

 clay shale and fine sand. The purely diatomaceous shale is 

 much less prominent than in other areas where the Monterey 

 group abounds. In the lower part of the group there is a 

 large percentage of clay shale interstratified with siliceous, 

 bituminous layers, while higher up organic materials are not 

 prominent. In the northern part of Sec. 28, T. 28 S., R. 14 

 E., the lower part of the group consists to a large extent of 

 light pumiceous rocks which are probably in part of volcanic 

 origin. With this exception the Monterey rocks are largely a 

 mixture of fine sandy clay and siliceous organic materials. The 

 rocks of this type outcrop almost entirely across the San Juan 

 district, in a zone parallel with the Temblor strata. To some 

 extent and in some places they are covered by deposits of later 

 age. 



