144 GEOLOGY. 
stances, they expand so as to break up into many shallow channels, and even to become entirely 
lost by evaporation or absorption in the sand and gravel of the plain. The San Joaquin rises 
in the heights of the Sierra, and flows westerly at right angles to the axis of the valley, like its 
tributaries, until it reaches the centre of the valley, and then flows northwest in the direction 
of its length. 
South of the San Joaquin, there are several large streams flowing from the Sierra Nevada into 
` the Tulare lakes. The lakes are broad, but shallow, sheets of water, with shelving shores, so that 
a slight increase of the volume of the water during the rainy season covers a large area of the 
surface. When the water is very high it is said to flow into the San Joaquin, thus connecting 
the two valleys by drainage. 
The valley of the Colorado Desert is, in many respects, similar to the Tulare Plains, but is 
more heated, arid, and desert-like. It is properly the northern prolongation of the valley of the 
Gulf, reaching from its shores to the base of San Bernardino, and bounded on one side by the 
Peninsula Sierra, and on the other by the ranges extending from the Gila to the mountain 
of San Bernardino. Its length to the head of the Gulf is thus about 180 miles, and its average 
breadth about 50, giving for its area 9,000 square miles. Its southern portion, or nearly half 
the area, is beyond the southern boundary of the State, and within the limits of Lower Cali- 
fornia. The elevation of this broad, plain-like valley is very slight, and a portion of its surface 
is probably below the level of the sea. It thus constitutes an important feature of the relief 
of the surface of the State. It is without any rivers, and only one or two small streams reach 
its borders from the Pass of San Barnardino and the Peninsula Sierra. These are speedily 
absorbed in the sand or evaporated. During seasons of very high water in the Colorado there 
is an overflow, which, extending inland to the lower parts of the Desert, forms & stream called 
New River. 
The trend of the longer axis of the plain or valley is: nearly — and southeast, being 
parallel with the mountains on each side, and coincident with the direction of the plains of the 
Sacramento and San Joaquin. The elevation of this valley presents a striking contrast with 
that of the coast slope on the opposite side of the mountains at the same distance from the 
crest. There, the elevation of the slope is not less than 1,000 feet, while the surface of the 
Desert is but little above the sea, and, in some places, is below it. But the contrast is still 
more striking when we compare the elevation of the Desert with the general surface of the 
Great Dasin, on the other side of San Bernardino, its average altitude being about 3,000 feet. 
Thus the conditions of elevation characterizing the opposite sides of the Bernardino Sierra are 
completely reversed in those of the Peninsula Sierra. In the former, the interior plains are 
most elevated ; in the latter, they are the lowest. San Bernardino is thus an interesting point 
in the physical geography of the State. It stands as a dividing pillar in the approaching angles 
of three broad areas of unequal elevation, and as dissimilar in their climate and productions. 
