18 GEOLOGY — SUISUN BAY. 



the sea level. Of its geological structure no account has yet been given, and it was not ascended 

 by our party. Its base, and perhaps its principal mass, is composed of the series of sandstones 

 which have been so fully described in the preceding pages. Specimens brought from the higher 

 positions of the mountain, which I had the pleasure of examining, show that with tertiary 

 sandstones, trap, serpentine, and diallage occur, but on what relative positions and quantities 

 I could not ascertain. Its structure, however, can scarcely be a matter of doubt. It marks a 

 conspicuous focus of action in the elevation of the Mount Diablo or Contra Costa range, which 

 we know has the same general structure as that of the range immediately bordering the coast ; 

 having probably the same date, and, like the coast mountains, having not constant but charac- 

 teristic axes of serpentine, flanked by thick beds of tertiary sandstone and shale, frequently 

 associated with which are jasper, diallage, and actinolite. 



Some of the tertiary beds of Mount Diablo are highly fossiliferous, containing the same 

 assemblage of genera, and probably of species, which are so characteristic of the coast moun- 

 tains in southern California. Among the most striking of these fossils is an immense oyster, 

 which is apparently the same with that figured and described p. 72, Plate IV, fig. 17 and 17a, 

 of this report, and called by Mr. Conrad Ostrea Titan. 



Gold has been found in small quantities in the streams flowing from Mount Diablo, and it 

 was at one time supposed that the diallage brought from there contained this metal, and a 

 quantity was transported to San Francisco, for the purpose of extracting the gold from it. Little 

 or none was found, however, and the error of the first experiment is said to have resulted from 

 employing mercury which had been before used in extracting gold, not all of which had been 

 separated from it. 



SUISUN BAY. 



North of Mount Diablo the range of mountains which has received its name is somewhat 

 divided and broken. This has offered a convenient avenue through which the drainage of the 

 upper country could be carried ; and through this the Straits of Carquines have been cut. The 

 waters of the great Californian valley, somewhat impeded in their flow to the ocean, and falling 

 to high tide level long before reaching the Golden Gate, accumulate above the straits, forming 

 Suisun bay, and the tule marshes at the junction of the Sacramento and San Joaquin. 



From Benicia to Goodhues, four miles, the north shore of Suisun bay is formed of the 

 sandstones which have been so fully described. 



Calcareous tufa. — Near Goodhues occurs a deposit of calcareous tufa, which has been quarried 

 out and burned for lime. It is very pure and white, and would undoubtedly make an excellent 

 mortar. 



From this point to Cordelia, the hills bordering the bay are all composed of some kind of 

 volcanic rock. The most abundant forms are soft and tufaceous, reddish or bluish gray ; other 

 portions are harder, either scoriaceous, compact; or vesicular trap. 



Above Cordelia, on Suisun creek, the rock which forms the hills, bordering the road, is a 

 hard, tough, umber-colored trap, which seems to be the prevailing rock over a large area in this 

 vicinity. 



It would make a very good building stone, and, though wrought with more difficulty than 

 the Benicia sandstones, would be far more strong and durable, and to many eyes more pleasing. 



Soil. — The soil of the shores of Suisun bay is black, and evidently highly productive, but 

 the area of level land below Cordelia is relatively small, and of that a considerable portion is 



