26 University of California Publications in Geology L VoL 11 



here the rock is not so well exposed, due to partial weathering of the 

 matrix. 



There are several bands of conglomerate from one to three feet 

 thick, interbedded with ribs of sandstone (plate 2b). In these sand- 

 stone partings there may be an occasional isolated pebble, entirely 

 surrounded by medium grained sandstone. The matrix of the con- 

 glomerate and the interbedded sandstone is typical Franciscan arkosic 

 sandstone. It is a hard, bluish-green rock with a quartzite-like frac- 

 ture. It contains specks of carbonaceous material and also flakes of 

 shale, which are sometimes so abundant that the rock is fissile when 

 slightly weathered. The matrix is cut by numerous thin veins of 

 white quartz. The boulders include various lithologic types and are 

 well rounded, the exceptions usually being shale boulders. 



The shale boulders are usually subangular, showing somewhat 

 rounded corners, but a few are decidedly angular. Generally they 

 are rather flat and are often arranged in bands. The photographs 

 show their character (plates 2a and b). In some places they overlap 

 each other in the peculiar way characteristic of stream gravels. 

 Usually the shale boulders are of dark or black color, but occasionally 

 they are gray. Many show a striping, due to the presence of fine 

 sandy layers, which probably represents the original bedding ; the 

 stripes are generally inclined to the sides of the boulder. 



This conglomerate also contains boulders of sandstone. These are 

 lithologically identical with the matrix in which they are embedded. 

 They are easily seen for the reason that they show a slight lamination 

 which is different in direction from the lamination of the matrix. 

 Many are cut by veins which end abruptly at the margins of the 

 boulder, having been formed before its detachment from the parent 

 rock. 



Igneous rocks of various types are well represented among the 

 boulders. Some are diabase which resembles certain phases of the 

 Franciscan diabase. A few rounded boulders of a rock containing 

 large bastite pseudomorphs may also be noted. The boulder shown 

 in the photograph directly above the notebook, is of this type 

 (plate 2b). 



The conglomerate contains boulders of black chert and also numer- 

 ous boulders of green radiolarian chert. There are some pebbles of 

 white quartz. Boulders of schist are fairly common. A few boulders 

 of green schist occur, and four or five boulders of glaucophane schist 

 were collected in a short time. 



