1918] 



Davis: The Franciscan Sandstone 



25 



Conglomerates 



The conglomerates of the Franciscan sandstone occur in lenses 

 at many horizons and are one of its most interesting features. The 

 matrix of the conglomerate consists of medium fine grained sand- 

 stone, in all respects like the ordinary sandstone of the Franciscan. 



The pebbles and boulders may be divided into two classes : those 

 derived from older formations, and those which appear to have been 

 derived from the Franciscan itself. Those of the first class are almost 

 invariably well rounded and range in size from less than an inch up 

 to a foot in diameter, the usual size being from two to three inches. 

 They comprise various lithologic types. Pebbles of black chert, unlike 

 anything in the Franciscan, are very common. There are also 

 boulders of granite, quartzite, and porphyritic igneous rocks of vari- 

 ous kinds. 



Those of the second class, which are apparently derived from the 

 Franciscan itself, show about the same range of size as do the boulders 

 of the first class. They may be rounded, but are usually subangular ; 

 some are decidedly angular. They comprise black shale, identical 

 in appearance with the black shales of the Franciscan, red and 

 green radiolarian cherts that cannot be distinguished from the red 

 and green cherts of the Franciscan, arkosic sandstones, containing 

 shale flakes and identical with the typical Franciscan sandstone, 

 diabase, serpentine, and glaucophane schist. 



In the Franciscan area of Mount Diablo there is a conglomerate 

 which is of interest since it contains many boulders of glaucophane 

 schist. This conglomerate outcrops at two points, within a course of 

 half a mile. The best locality is in Section 5, Township 1 South, 

 Range 1 East of the Mount Diablo Base and Meridian. The point of 

 the outcrop lies about 0.54 mile south and 1.45 miles east of Mount 

 Diablo. The second locality lies southwest of the first one and is in 

 the northwest corner of Section 8, about one mile east and 1.15 miles 

 south of the South Peak. 



At the first locality the stream falls over a cliff about fifty feet 

 high. Back of the cliff the stream has cut a sharp notch in the solid 

 rock from eight to ten feet deep. In the bottom of this notch there is 

 a pothole four feet across, and at least five feet deep, the bottom being 

 filled with stream pebbles. The stream has scoured the walls of the 

 notch and the pothole, and in the clean, fresh surfaces an excellent 

 section is presented. The second locality is an outcrop on a ridge ; 



