Clark: The San Lorenzo Series of Middle California 



85 



feet of medium-coarse, gray, micaceous sandstones, interbedded with 

 which are minor layers of softer sandstones and clay-shales. Reddish- 

 brown, elongate concretions, which in some cases are two to three 

 feet or more in length, are scattered in fairly definite layers through- 

 out. The sandstone is arkosic and contains a great abundance of 

 fairly large flakes of white mica, some of which are almost a quarter 

 of an inch in diameter. The sandstone, in places, is loosely consoli- 

 dated and certainly has not been subjected to metamorphism. Tln j 

 arkosic character, together with the great abundance of the mica, 

 apparently shows that these sediments were derived in large part 

 directly from igneous rocks. 



Above the seven hundred feet of micaceous sandstone is about two 

 hundred feet of soft brown carbonaceous shale. This grades up into 

 a series of alternating sandstones and shale, the thickness of which is 

 about 1400 feet. The sandstones, for the most part, have the same 

 general character as the basal sandstones. Very few exposures of the 

 shale were found ; usually it is brown to gray, argillaceous in char- 

 acter, and contains carbonaceous material in considerable abundance 

 The uppermost bed of this general unit of the section is a medium 

 brown sandstone. 



The sandstone just mentioned apparently grades up into a series 

 of shales which, at the base, are rather dark and argillaceous and con- 

 tain an abundance of carbonaceous material ; at about three hundred 

 feet from the base, the shale is lighter in color and much harder, and 

 the outcrops are fairly conspicuous. The thickness of this part of 

 the section is about one hundred and sixty feet. The shale of this 

 horizon varies from a chocolate color to almost a pure white ; some 

 of the layers, especially near the base, are rather thick and massive ; 

 higher up they are thinner-bedded and in places form typical paper 

 shale. Marine diatoms are abundant in the lower beds, the genus 

 Coscinodiscus being the most common form. Impressions of leaves 

 and rushes are rather common in the upper part of this shale member 

 and diatoms are conspicuous by their absence. 



The light-colored shales, just described, grade up into soft carbon- 

 aceous shah j s with alternating layers of rather coarse, micaceous 

 gray to yellowish brown sandstone. The sandstone layers predom- 

 inate toward the top. The different shale layers vary from a few 

 inches to several feet in thickness; some of the layers are white and 

 fairly hard ; some are almost black, while others are reddish brown 

 in color. Leaf impressions and carbonaceous material are abundant 



