Ib 



northward, the coast line of mountains to Fort Ross are of the same primitive 

 character, being made up of rocks of the granite series, imposed upon which are 

 those of sedimentary origin. 



The latter class consists for the most part%f a coarse conglomerate, alternated 

 at distant intervals with small beds of the coarse brown and yellowish sandstone 

 incident to almost every other part of the coast line from one extreme of the State 

 to the other. The latter beds contain marine mollusca in the fossil state, but none 

 were observed in the conglomerates which lie superior to them. At Bodega Head 

 several beds of these conglomerates are found ; and the point forming the reef at 

 this place is of the same character. 



There is reason to believe that this conglomerate is auriferous ; and subsequent 

 to my visit to these localities gold was found in small quantities among the debris 

 of these rocks. At Point Tomales on the west shore of the bay, gold also abounds, 

 but in small quantity. It is found about one mile from the northern end of the 

 point, near the settlements on that shore. 



Leaving the coast line and passing into the interior, we find the geological char 

 acter of the country changing in a material degree. At the distance of twenty 

 miles from the coast and east of Fort Ross, the volcanic rocks again make their 

 appearance, and continue northward and eastward almost uninterrupted for sixty 

 or seventy miles. Occasionally an isolated group of some of the primitive series 

 is met with in these ranges, and when this is the case more or less of the marine 

 tertiaries accompany them. All that were met with have apparently the same 

 relative age as those of the Monte Diablo groups. 



Aniong the mountains on the south-west and west of Clear Lake, there are large 

 masses of scorial associated with alsidian, (volcanic glass), which is spread broad 

 cast among these hills, thrown out from recent craters, often met with in this part 

 of the State. 



Among the basalts of the country south and about the Petaluma district, no 

 scoria was found in place that had the appearance of having been ejected in the 

 vicinity, but a plentiful supply is found upon the surface, all of which are evidently 

 transported masses from the more northern sections. 



At first it is somewhat difficult to account for these features in the lower lands 

 of this plain, as there seemed no adequate cause for producing so extensive a local 

 deposit, as no river runs through this valley at the present time. In some exam 

 inations made in the earth thrown from a well on the premises of Judge Thom 

 son, the secret was in some measure explained; for here the shells of several fresh 

 water molluscas of the genus unio, and parts of a crustacean, (probably astacus), 

 were found at the depth of eighteen or twenty feet below the surface. 



On subsequent examination of the plain, an old river bed was traced for nearly 

 its entire length, having two distinct terraces on each side. This ancient bed was 

 followed to within about two miles of the present bed of Russian River, from 

 which it is divided by a very low range of hills, and here the former bed is 

 entirely effaced. 



Near this point, (which is called Russian River Valley,) the Russian River 

 Canons suddenly breaks through the primitive rocks in a broad chasm, the bend 

 at this place forming almost three sides of a parallelogram. 



I have no doubt from the present position of the stream that it has been divert 

 ed from its course, which was originally through the great valley of the south, 

 and forced into its present bed during those violent convulsions that, at a compar 

 atively recent period, must have disturbed this entire section. A stream of the 

 capacity of the above would be amply sufficient to account for the local drift of 

 this plain, and which cannot be well explained on any other hypothesis. 



On the eastern side of the Petaluma Valley, a range or spur of low moun 

 tains separates the former from the Valley of Sonoma. These mountains are made 



