GOLD—GRANITE-PLAIN OF BASALTIC LAVA. 
21 
A ..out one mile up the river above the fort, the granite passes into gneissoidal and slaty beds 
containing much hornblende. These beds all have a trend of N. 40° to 45° W., and they dip 
to the eastward at a high angle. The slaty masses alternate with the more compact and 
harder granite. This difference of mineral constitution and aggregation of the rock causes it 
to wear away unequally, and portions of the hills are rapidly decomposing under the influences 
of the weather. They thus cover their steep slopes with a rich soil, which ustains a growth 
of oaks and pines. 
View No. 1, forming the frontispiece to the report, shows a valley between the granitic and 
metamorphic hills on the north side of the river. The sharp edges of the gneiss or mica-slate 
stand out from the surface in many places in large tabular masses. 
Plain of basaltic lava .—Near the fort, one of the granite hills that rises to an elevation of 
about one thousand feet above the river is capped by a horizontal layer of compact basaltic lava 
about one hundred feet thick. It presents a bold escarpment fronting the valley, and from 
below looks like a grand fortress with vertical walls and rounded bastions. 
On ascending the summit of this rock, it is seen to extend for great distances, apparently in 
a perfectly level plain miles in length and breadth. It borders the San Joaquin in its circuitous 
course for many miles ; and continuations of the same broad field can be seen on the opposite 
side of the stream. A partial bird’s-eye view of a part of this lava plain is presented in the 
wood-cut. The sketch was taken from the edge of the plain above the military station, and 
includes the gap through which the San Joaquin flows. The summit of the Sierra is shown in 
the distance. 
It will be seen from the engraving that the exposed edges of the sheet of lava do not in all 
cases present one continuous vertical wall, but rise in a terraced form, indicating the existence of 
three or more parallel sheets or layers, one superimposed on the other. These layers, or tabular 
masses of rock, exhibit, in a rude manner, the columnar structure, characteristic of similar 
igneous rocks. The weathered surface of this rock presents a rusty brown color ; but the inte¬ 
rior is black, and shows a crystalline structure. Loose blocks, when struck by the hammer, 
give a clear ringing sound, and break with a conchoidal fracture. Other parts of the rock were 
more scoriaceous, and filled with small cavities that do not appear to have contained minerals, 
but are probably air-cells. The rock is highly ferruginous, and affects the compass needle very 
strongly ; in some places showing polarity. 
In many parts of the surface I observed a distinct ridge-like character of the rock ; there were 
distinct vertical planes, having a parallelism of direction, along which the rock cleaved with 
greater facility than in other lines. At other portions of the surface, that had been exposed to 
atmospheric changes for a long time, a distinct linear arrangement of the minerals was evident 
