12 



COUNTY OF SONOMA. 



From the known character of the rocks, both primitive and sedimentary, th* 

 abound along the shores of our large bays, and from thence extend to some dis 

 tance into the interior, it might at first be thought that an easy inference could 

 be drawn of the character of the country to the north-west having so many of 

 the general external features that appear in preceding districts but such is not 

 the fact, for soon after leaving the coast line of the bays, we enter a widely dif 

 ferent character of rocks from those found near them on the head of the San 

 Antonio, the serpentine and other primitive rocks which are found from this 

 point southward, cease. The country from thence northward, to the town of 

 Petaluma, assumes a new aspect, the rocks changing from the primitive groups 

 to those of recently volcanic origin. So far as my examinations extended, these 

 latter did not in any case present the slightest trace of any one of the sedimen 

 tary rocks, excepting the local drifts. 



The entire group which underlies this part of the country is of a basaltic char 

 acter exclusively, having in some cases the massive form, in others again the true 

 columnar structure. At the distance of about one mile westward of the town 

 of Petaluma the latter characteristic may be conveniently examined, for at this 

 point the basaltic columns are being quarried for the purpose of erecting build 

 ings at this flourishing place. At this locality the prisms are usually about six 

 feet in length each, and regularly pentagonal in form, standing upright, and on 

 a prism of equal diameter in breadth. This enables them to be thrown down 

 easily, after which they are cut into sections of the required thickness", and used 

 for the purpose above named. 



This group of rocks extend in a westerly direction, to within three or four 

 miles of the coast, but are not found upon the shore line, with the exception of 

 one instance, and this occurs about six miles south-east of Lopes Embarcadero, 

 on Tomales Bay. 



In a northerly direction and among the high hills skirting the valley of Peta 

 luma, these rocks are traceable for thirty or forty miles, after which they give 

 place to the primitive rocks again, which continue on the ridge fronting the 

 coast for fifty miles further on towards Mendocino. 



To localize more particularly the boundaries of the volcanic rocks westward 

 from Petaluma, it will be necessary to state, that near the crossing of the Estero 

 Americano, on the Bodega Road, the first out-cross of the primitive series is found, 

 surmounted by the same fossiliferous rocks as those of Monte Diablo. This is the 

 first instance in which the latter rocks are met with on this ifne of travel. It is 

 interesting to observe the regularity that presents itself in these particulars, for 

 this deposit of marine fossils fixes the relative age of these distant points, and 

 shows us more plainly their cotemporaneous origin. The primitive rocks and the 

 fossiliferous group resting upon them are tilted to the west, and at the contact of 

 the volcanic and primitive groups the most unmistakable evidences of change in 

 structure is apparent. 



In view of these facts it may not be amiss in reasoning upon this subject, and 

 in considering the relative ages of plutonic rocks, not to place too strong a stress 

 on more isolated facts, which might lead us to suppose that a primitive group may 

 be of more recent age than those of known modern deposits. 



The basaltic rocks of this district are clearly post-tertiary, but I am inclined to 

 the belief that the statements made, of the primitive rocks of San Francisco being 

 of that age, will admit of some argument, though I do not design to discuss that 

 question in this place. 



From Tomales Point (which is of granite), southward, and from the same point 



