22 
GEOLOGY. 
from the belted and striped character of the surface. Masses broken from this place verifi .1 the 
surface indications ; the imbedded crystals being found to be arranged in parallel lines , and with 
the broad tabular surface of the crystals all in parallel and vertical planes. This linear arrange¬ 
ment of minerals in volcanic rocks, and especially their occurrence in vertical planes in hori¬ 
zontal beds or sheets of basalt, is exceedingly interesting, as it tends to throw light upon the 
origin of the laminated structure developed in granitic rocks. 
The path leading up to the surface of this table-land was along a valley at the southeast of 
the fort, and was accessible to mules. I was able, with a little difficulty, to get my mule up 
the cliff at a point where it was much broken down by weathering. I rode for several 
miles over the level plain without finding any obstruction or much soil to obscure the black 
floor of lava. The plain appeared unbroken far into the distance ; hut I soon came upon a 
ravine with abrupt sides, visible only when the brink was nearly reached, and was forced to 
return. The plain is, doubtless, intersected in this manner by ravines invisible from a dis¬ 
tance, unless they extend in the direction of the line of vision. Looking off from the edge of 
this plain towards the San Joaquin, a long line of flat-topped hills was visible. These hills are, 
doubtless, portions of the table-mountain seen on approaching the river. The summit appears 
to have been continuous, and is intersected by gaps so as to form a series of flat-topped mounds, 
separated by little valleys. The original plain probably inclined gently towards the west, as 
an uniforn inclination is now distinctly visible in all the summits. The San Joaquin was seen 
winding around the southwestern point of this interesting range. 
The immediate contact of the basalt with the subjacent rocks was not visible on the slope 
facing the San Joaquin. I ascended on that side and traced the granite nearly to the lower 
line of the basalt. The upper portions of the granite showed horizontal beds, differing in their 
mineralogical character ; a portion was very coarse-grained and highly feldspathic, other 
portions were finely laminated, and with these was a bed consisting of large crystalline masses 
of albite imbedded in quartz. These rocks were, however, exposed for a short distance only, 
and may be portions of large granitic veins. 
The general character of the granite in the vicinity and lower down the slope is compact and 
even in texture. On descending from the plain of basalt at the southern margin, I found that 
there was a vertical exposure of the edges of horizontal sedimentary beds underlying the 
basalt. These were composed of gravel, fine white sand, white clays, volcanic sand, and great 
masses of light and friable pumice stone. These masses were of various sizes, and formed layers 
over one foot thick. Volcanic material seemed to abound throughout the beds, and formed the 
principal part of their mass. 
The whole thickness of the exposure did not exceed 100 feet, and a great part of it was so 
much obscured by soil that its character could not he satisfactorily determined. The relative 
positions of the basalt, sediments, and underlying granite, will be better understood by refer¬ 
ence to the accompanying outline section, representing the bed of the San Joaquin and the hills 
on each side. 
SECTION OF THE VALLEY OF THE SAN JOAQUIN AT FORT MILLER. 
These underlying sediments are probably members of the same series that form the isolated 
