Doc. No. 9.] 35 



of that character exclusively.* In some parts of the country^they form 

 beds of considerable thickness, and are rich in organic remains. In 

 some parts of the mountains, the shells are of present existing species, 

 not found upon the coast at the present time ; these consist of three or 

 four species of the Area, and one or two of the Pectinea ; these rocks 

 are commonly found resting upon the primitive series, though at times 

 on some of the more recent igneous intrusions. It is considered very 

 doubtful by our conchologists in this part of the world, whether the 

 living shells of the above species now inhabit these waters ; and yet 

 these imbeded remains are found in abundance on the summits of our 

 hills, removed many miles inland from the element they formerly in 

 habited. 



The fossils of this part of the Coast Mountains, and which now ap 

 pear to be extinct, consist of three species of the gryphae, two at least, 

 of Pectinea, Astarte and Cytherea, the species of which will be noticed 

 more in detail in another part of this report. 



The position of the miocene rocks in the northern part of these moun 

 tains, appears to be directly succeeding the primitive ; yet it is found 

 often that they rest upon the trachytes; in all cases that have met my 

 observation, the latter rocks when thus found in contact with the sedi 

 mentary group of this period, have broken through fissures in the prim 

 itive formations, subsequent to the elevations which occurred during 

 the miocene and middle pliocene periods, or perhaps during the deposi 

 tion of the latter. The trachytic rocks north of Napa, where they 

 leave the sedimentary group, gradually pass into vesicular lavas, and 

 in these sections considerable quantities of obsidian are to be met with ; 

 the Indians in the vicinity of Clear Lake use it for the manufacture of 

 their arrow-heads. 



Again, on the shores of Suisun and San Pablo bays, and on the 

 Straits Carquinez, these igneous rocks are found injected into the seams 

 of the sandstone, contorting the strata to a considerable degree. West 

 of these bays tjie primitive rocks occur at intervals for several miles, 

 and the fossilifemus rocks are found holding the same position as those 

 further north. These alternate intrusions of the trachytic rocks con 

 tinue south among the mountains on the east side of the Bay San Fran 

 cisco, and for thirty -five miles beyond its southern extremity ; after this 

 they pass into porus lava, and compact masses, ceasing to present any 

 traces of fossiliferous deposits resting upon them for forty miles on this 

 western ridge ; after this they again (the fossiliferous) make their ap 

 pearance in small detached masses, which gradually increase to a well 

 defined formation beyond. 



These rocks in many instances are found to rest upon the recent vol 

 canic series direct, and in these cases we find, as may be expected, a 

 corresponding change in their structure, often to an extent that has ob 

 literated every trace of organic remains ; while in other cases, where 

 the fossiliferous group has been protected from the more direct influence 

 of the later igneous series by the intervention of any of the primitive 



** The lacustrine deposits discovered during the past summer by Mr. "W. P. Blake, on the 

 desesrt of the Colorado are the first fresh-water formations found in this country. 



