16 



GEOLOGY VICINITY OF BENICIA. 



as to determine the nature and relative position of all the rocks which are exposed there ; nor 

 was I able to carry my observations uninterruptedly from the section exposed on San Pablo bay 

 to that of the straits. The shores of the upper part of the straits I found to consist of a great 

 number of alternations of thin-bedded argillaceous sandstones, very similar to some of those 

 exposed on the south shore of San Pablo bay, and precisely like those of the western shore on 

 Point San Pedro, above San Kafael, and those of Rincon point, south of San Francisco. 



These sandstones are inclined at a very high angle, some of them being nearly vertical, but 

 not having a unifoim dip. One, and perhaps more than one anticlinal crosses the straits in 

 the general trend of the range. I was not able to detect any fossils in the rocks there exposed, 

 but have no doubt of their identity with the San Francisco group, having the same geological 

 age with those of San Pablo bay, and therefore Miocene or more recent. 



I saw no evidences here or below of the existence of the older slates which have been said 

 to exist in this vicinity. Nor did I see here, or anywhere on the shores of San Francisco or 

 San Pablo bays, with the exception of a single locality, the Golden Gate, any trap, trachyte, 

 or other unmistakable plutonic rock. 



The greenish argillaceous sandstones of the Straits of Carquines have threads and sheets of 

 gypsum running through them in all directions, a feature shared by the similar strata in the 

 localities which I have mentioned, Point San Pedro, &c. 



BENICIA. 



The geology of the vicinity of Benicia is apparently but a continuation of that of the Straits 

 of Carquines. An anticlinal crosses from near Martinez to the vicinity of Navy Point, on either 

 side of which are found sandstones, shales, &c, the apparent equivalents of the San Francisco 

 group. There is, however, a conglomerate at Navy Point which presents somewhat different 

 characters from those of San Pablo bay, the pebbles which it contains consisting in a great de- 

 gree of fragments of the harder silicious rocks, jasper, hornstone, agate, carnelian, &c. 

 Though I had little opportunity of examining it, my impression was that it did not present a 

 new element in the geology of our route. 



At Navy Point Mr. W. P. Blake, geologist of the Pacific Eailroad Survey in southern Cali- 

 fornia, under Lieut. Williamson, U. S. A., discovered teeth of sharks, which give to these strata 

 a date certainly no older than the Miocene. I did not notice any fossils in any of the rocks about 

 Benicia ; but it is probable they would reward a more thorough search than I was able to make. 



SANDSTONE MILLS NEAR BENICIA. 



Sundsl-07ies. — The high hills which border the straits and occupy all of the area between 



