Doc. No. 9.] 27 



natural laws that are found to prevail in other parts of the country in 

 relation to auriferous veins. 



These sand-stones are found to extend into the interior in a northwest 

 direction, a distance of about eight [miles, when they are succeeded by 

 volcanic rocks of recent date for thirty miles in the same direction ; the 

 points of contact in the igneous and sedimentary rocks exhibit distinct 

 and striking marks of change in structure among the latter in many 

 places, and it is not uncommon to find trachytic injections into the 

 sand-stone along the line of coast on the bay and straits. 



Where these veins are large, the sandstone in contact is often nearly 

 as hard as the trepan rockf' itself, a true conchodial fracture, having q_uite 

 sharp or roughened edges, with a semi-earthy surface, is the result of 

 this metamorphosis. Above this sandstone and resting directly upon it, 

 is a littoral sea-beach, having an elevation of about thirty feet above 

 high tides, and extending for several miles along the coast of the bay. 

 It is composed of fragmentary and entire shells, mixed with a little sand 

 and clay ; its thickness varies from one to three feet. Its position is im 

 mediately below the alluvium. 



It forms a distant white line along the bay coast of San Pablo on its 

 north side for eight miles, and may be seen at the town of Benicia in 

 the vicinity of the sandstone quarries at that place. The shells of which 

 this beach is composed consist of a small species of ostrea, purpura, and 

 other small shells now inhabiting these waters. The elevation of this 

 beach points to a period comparatively recent, when subterranean forces 

 were in operation in elevating the lands adjoining the coasts and bays, 

 which part of our subject will be considered more in detail when re 

 viewing the geological changes which have occurred in the different por 

 tions of the coast-line followed. 



Among the sandstones of this region is a bed of limestone having an 

 average thickness of two feet ; it is found one mile north of the town. 

 This limestone was traced in a northeast direction for two miles, and is 

 probably the same range as observed by Mr. Tyson on the east side of 

 the hills on Suisun Bay. 



The foot of the ridge lying between Suisun and Napa Valleys, was 

 followed for twenty miles. At this distance from the bays it attains an 

 altitude of about twelve hundred feet. The rocks composing this ridge 

 are mostly volcanic, with sandstone on their eastern flanks dipping east 

 towards the Sacramento Valley ; a few very imperfect casts of marine 

 shells were observed, but none sufficiently perfect for preservation. 



The ridge on the west border of Napa Valley presents much the same 

 characteristics as those on the east. These two ridges unite about eight 

 een miles north of Napa City, at which place they become extremely 

 rugged and elevated. 



The highest and most conspicuous peak in this range is Mount Helen ; 

 its sharply defined outline and truncated summit shows most conclusive 

 ly its volcanic origin. A section cutting Napa Valley nine miles south 

 of the town was made, and carried across the Sacramento Valley to the 

 foot-hills on its eastern side, by which the grade of both valleys and the 

 altitude of the hills were obtained. 



Before reviewing the geological changes that have occurred in the 

 Sierra Nevada, Monte Diablo and Coast Mountains, a view of that part 





