MOUNT DIABLO—DIALLAGE—DEEP SOIL. 
7 
continuous from Martinez southward to Livermore’s pass, and beyond it. It probably form s 
the basis of all the rounded bills along the valleys already followed by the Expedition. Where- 
ever the strata appear, they are highly disturbed, the angle of dip being generally over 45°. 
The general or average trend may be said to be from 30° to 45° west of north and east of 
south. Several long ridges, presenting precipitous faces towards the southwest, were visible at 
the base of Mount Diablo ; they were seen from a distance of several miles, but they apparently 
had the general trend of the hills we followed, from northwest to southeast, and were of strati¬ 
fied rocks. 
Mount Diablo is probably of igneous origin, but I could not observe any plutonic or volcanic 
rocks along the road we traversed. The rugged character and loftiness of the peak indicate 
that it is composed of hard crystalline minerals. This view is supported by the character of the 
rock brought from near the summit, of which a specimen was obtained. It is a mass of crystals 
of diallage or bronzite, confusedly mingled together, but possessing ail the characters of the 
mineral. It has a brown color and bronzy lustre, and cleaves very readily, with broad and 
brilliant faces. 1 It is common in serpentine, and it is most probable that this is the summit- 
rock of the mountain. 
The soft sandstones and earthy strata forming the hills of the part of the route under conside¬ 
ration, are easily acted upon and worn away by the weather, and thus furnish the material for a 
deep and loose soil. This is readily acted upon by the rains, and smooth rolling hills result. 
DEEP SUN-CItACKS IN THE SOIL. 
The soil so produced contains a large amount of clay, and it shrinks so much by drying during 
the summer, that the surface is traversed in every direction by cracks and fissures, that are 
frequently two inches wide and sixteen or eighteen inches deep. Their character and form is 
represented by the figure. 
When this deep soil becomes saturated with water during the long heavy rains of winter, 
these fissures close. At such times, an immense weight of water is absorbed by the dry soil; 
and when it rests on the sides of the steep hills, or upon an inclined surface of rocks, large 
bodies of the saturated earth become loosened by the great weight, and slide down in an aval¬ 
anche of mud to the valley below. It is sometimes the case, that an area of fifty or one hundred 
feet in diameter, will break loose near the summit of a hill, and slide down over the surface 
below it without breaking the soil; but the production of a complete excavated track from the 
top to the bottom of the hill is more common. This tearing up of the soil prepares the way 
for a current of surface water, which soon wears deep ravines in the yielding materials. 
The soil appears to be well adapted to the growth of grains. The hills are covered during the 
spring and early summer with a luxuriant growth of the wild oat ; and wherever the valleys 
1 A large quantity of this rock was quarried a few years ago, under the supposition that it contained a large per centage 
of gold. If the same as the specimen I have, mere inspection by a competent geologist would have shown the fallacy of the 
belief, and saved much expenditure of time and money. 
