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University of California Publications in Geology [Vol. 11 



Nature of Contacts Between Chert and Sandstone 



In mapping, it is usually easy to distinguish the line of contact. 

 The sandstone generally weathers easily, giving rise to soil with 

 smooth, grassy slopes. The chert areas even on gentle slopes are 

 usually covered with a thinner and poorer soil in which are embedded 

 large numbers of chert fragments ; where the chert is massive there 

 are numerous bold outcrops. As a rule there is a thickness of several 

 feet of shale between the sandstone and the chert. This shale is soft 

 and easily affected by the weather, so that it presents no outcrops. 

 Readjustments between the two formations in this zone of soft rock 

 have, in many instances, caused the shale to be greatly sheared and 

 disturbed. Owing to these circumstances one finds the best sections 

 across the contact in artificial cuttings or in cliff sections along the 

 coast. 



Near the end of the Twenty-fourth and Hoffman street-car line 

 in San Francisco a section exposed in the side of a street-cut shows 

 the nature of one of these contacts. Here the Ingleside chert rests 

 upon the Marin sandstone. The sandstone at the base of the section 

 is of the ordinary type found in the Franciscan. This passes gradu- 

 ally into a soft gray shale about four feet thick. Above this is a six- 

 inch bed which resembles a greatly altered tuff. Above this are radio- 

 larian cherts of the usual kind. 



At Rock Ridge Quarry in Oakland, the contact of the Sausalito 

 chert and the Cahil sandstone is well exposed in the east wall of the 

 old quarry. At the base, there is a sandstone rather rich in carbon- 

 aceous material, which passes gradually into a gray shale of the ordi- 

 nary variety, about a foot thick. Above this is a fine grained, green 

 shale quite different from the gray shale beneath and exactly like the 

 green shales interbedded with some of the green radiolarian cherts. 

 The contact between the green and the gray shale is sharply marked 

 and the two may be easily separated with the point of a pick. The 

 green shale passes up into a red shale of the type found interbedded 

 with red radiolarian cherts. The change from the green shale to the 

 red shale occurs within an inch or two and consists simply in a grad- 

 ual transition in color. The red and green shales together make up 

 about two feet of the section, there being about a foot of each. The 

 red and green shales differ from the underlying gray shale in that 

 they are finer grained, more compact and a trifle harder. Immedi- 



