1918] Davis: The Badiolarmn Cherts of the Franciscan Group 243 



ately above the red shale there are red radiolarian cherts consisting 

 of alternating bands of chert and shale, having a total thickness of 

 four to four and a half feet. Above the cherts is a bed of fine red 

 shale, one to two feet thick, identical with the red shale below. This 

 bed is disturbed somewhat by a flow of pillow basalt which rests 

 above it. 



On the east side of Red Rock Island, in San Francisco Bay, there 

 is a contact between chert and sandstone. It has been intruded by 

 igneous rock and somewhat broken, but there are several feet of gray 

 shale between the sandstone and the chert at this place. 



At Fort Baker in Marin County, a contact is exposed in the mili- 

 tary road, which shows several feet of shale between the sandstone 

 and the chert. 



On the Twin Peaks Boulevard in San Francisco, just across from 

 the reservoir, a contact of chert and sandstone has been exposed in the 

 road cut. Here the shearing has been so great that the relations are 

 not very clear. The shale between the two formations is rather thick — 

 probably fifteen to twenty feet — and for the most part it is very fine 

 grained and unlike ordinary shale. 



In Perkins Creek on the north side of Mount Diablo, a more 

 abrupt contact may be seen. The cherts are dark gray to pale pink 

 in color. Between the cherts and the sandstone are five inches of fine 

 grained gray shale which represents the extent of the sedimentation 

 between the sandstone and the chert. 



Fairbanks 3 has described abrupt changes of sandstone to chert in 

 certain sections in the San Luis quadrangle. 



Occurrence of Chert in Lenses 



Beside the larger areas already mentioned, which mark out definite 

 horizons and constitute distinct formations, there are great numbers 

 of smaller bodies of chert exposed in the Franciscan terrane. These 

 appear as patches, irregularly distributed through the sandstone. 

 The cherts in these smaller masses are in every way like the cherts of 

 the larger formations. The size of these bodies varies considerably. 

 Some are fairly large, covering several acres, while many are small 

 and cover only a few square yards. Around these outcrops the soil 

 indicates the presence of sandstone. These isolated masses are very 

 numerous in certain regions. South of San Francisco, in the southern 



3 Fairbanks, H. W., San Luis Folio, U. S. Geol. Surv. Folio no. 101, 1904. 



