GEOLOGY MOUNT DIABLO. 17 



Vallejo and Benicia, and which, with their wavy and graceful outline and their unbroken 

 mantle of wild oat, present a view so peculiar and so pleasing, are all composed of the San 

 Francisco group of sandstones ; at least such was the inference which I derived from my ex 

 amination of them. Immediately back from the town of Benicia the sandstone is considerably 

 massive and thick bedded, and has been extensively quarried as a building stone, for which it 

 serves a very good purpose, but is open to the objections suggested when speaking of a similar 

 stone at San Francisco. When exposed in ledges it has a most striking resemblance to trap. 

 About Major Vaughn s rancho, two miles northwest of Benicia, where we were encamped for 

 some days, the only rocks visible are a soft grayish sandstone, generally somewhat massive, and 

 thin layers of greenish brown shales, some of which are soft, and where exposed are often covered 

 with an efflorescence of sulph. alumina. These beds are dipping in different directions in the 

 different localities where examined, and are evidently traversed by several lines of uplift. 



Five miles northwest from Benicia, among the rounded sandstone hills, the most elevated 

 summit in the vicinity is crowned by a crest of red jaspery rock, similar to that which occurs so 

 abundantly at Point Diablo, near San Francisco. This crest is particularly rough and rugged, 

 is nearly half a mile in length, and has the general trend of the ranges of hills which surround 

 it, and of the Mount Diablo mountains. It is somewhat cellular and spongy in texture, and pro 

 jects forty to sixty feet above the softer rocks which flank it. It exhibits no tendency to stratifi 

 cation, as far as I examined it, and has all the external characters of an injected dyke of plutonic 

 rock which owes its relief to the erosion of the softer material which once formed its enclosing 

 walls. The junction of the sandstones with the jasper is covered by debris and not visible; but 

 where exposed in the vicinity, though much disturbed, they exhibit no marks of metamorphic 

 action. 



Warm spring. At the north end of this ridge of jasper is a copious spring, which seems to 

 issue from immediate contact with the rock. The water is strongly charged with sulph. hydro 

 gen, and is slightly thermal, having a higher temperature at its source than in the basins which 

 it fills, a few rods down the hill-side. I had no thermometer with which to test its tempera 

 ture, but supposed it to be between 80 and 90. No silicious or calcareous deposit is made by 

 it. It has the common taste of sulphur water, and is habitually used by a family residing near. 



In a ravine near by, lenticular nodules of argillaceous iron ore are interstratified with the 

 thin layers of greenish, soft, fine-grained sandstone. 



Soil. The country about Benicia is generally productive, though its value as an agricultural 

 district, in common with the whole of the interior valley, is greatly impaired by a want of water. 

 The soil, which is formed by the decomposition of the argillaceous sandstones of the San Fran 

 cisco group, is dark, deep, and rich. The surface is thrown into hills, often rising several 

 hundred feet, but in gentle swells and slopes, with smooth and graceful curves, never presenting 

 a broken outline. And this surface, as far as the eye can reach, is covered with the wild oat, 

 (Avenafatua.) No peculiarity of the soil, but atmospheric influences have given to this region 

 such a prevalence of annual vegetation, and limited its trees to the few scattered clumps of 

 evergreen oak, (Quercus agrifolia, ) which, so much resembling orchards, combine with the 

 unbroken stretches of wild oat to give to it the appearance of being universally and thoroughly 

 cultivated. 



MOUNT DIABLO. 



While encamped near Benicia, this mountain, which we had first seen when entering the 

 G-olden Gate, was in plain view from our camp. Its altitude is 3,760 feet, while its base is at 

 3 Y 



