16 GEOLOGY. 
from the ocean. This range, like the two before passed, has nearly a NW. and SK. trend, 
and is composed of gray granite and mica slate, traversed by veins of felspar several feet wide, 
which are visible for long distances. The felspar is pinkish white, in large tabular crystals 
containing schorl. From the summit of this range we descended into the beautiful valley of 
Santa Isabel, which, surrounded by picturesque summits, with its green meadows and beautiful 
groups of oaks, presented a charming picture, contrasting strongly with the general sterility 
of the surrounding region. 
From ‘‘Warner’s Ranch’’ the road leads by a gentle ascent to ‘‘The Oaks,’’ the divide 
between the waters of the Pacific and the desert. From this point to Carisso creek we fol- 
lowed the windings of the draining stream, which gradually becomes more intermitting and 
alkaline, until it entirely disappears in the sands of the desert. Throughout this distance the 
geological features of the route remain constantly the same ; the aspect of the country becoming 
more barren and repulsive at every step. 
Tertiary strata.—Approaching Carisso creek, we saw, for the first time in many days, strata 
of unchanged sedimentary rock. These consist of shales and clays, of a light brown or pinkish 
color, forming hills of considerable magnitude at the base of the mountains. From their soft 
and yielding texture they have been eroded into a great variety of fantastic and imitative 
forms. This series of beds has been greatly disturbed, in many places exhibiting lines of 
fracture and displacement. Where they are cut through, in the bed of Carisso creek, they 
contain concretions and bands of dark brown ferruginous limestone, which include large 
numbers of fossils, Ostreas and Anomias. These have been already described by Mr. Conrad, 
and are considered of Miocene age. In the debris of these shale beds I found fragments of the 
great oyster, (Ostrea Titan,) characteristic of the Miocene beds of the California coast. 
A few miles north of this point similar strata, probably of the same age, were noticed by 
Dr. Leconte, but there they contain Gnathodon, an estuary shell ; showing that the portion of 
the desert where they are now found was once covered by brackish water. 
THE COLORADO DESERT. 
That which is called by courtesy Carisso creek is usually, throughout the greater part of its 
course, a dry gravelly or sandy trough, which in its form gives evidence of being at times 
traversed and flooded by a stream of water. It is the natural avenue of escape for the rain 
which falls over a large area on the eastern slope of the Peninsular mountains ; and along its 
rocky bed, beneath the sands, a meagre and alkaline stream at all times percolates, rising at 
intervals to the surface, and saving from absolute death those who are condemned to traverse 
this inhospitable region. This stream makes ite last appearance near the station-house at the 
edge of the desert, where, for a few hundred yards, it flows over the sands, encrusting them 
with the salts it holds in solution, and is then absorbed by the porous soil. The continuation 
of its bed forms a broad and dusty avenue, cut through the accumulations of debris washed 
down from the mountains above, and gives easy access to the plains which constitute the 
desert proper. At rare and uncertain intervals a wild and impetuous torrent, caused by some 
storm in the mountains, rushes through this excavated channel and spreads itself, with a vast 
- amount of gravel and sand, over the surface of the desert. Yet so infrequent and fitful are 
these supplies of water that they fail to redeem any portion of the wide area into which they 
flow from the stern and unrelenting sterility which there reigns supreme. 
The flanks of the mountains and the slopes of the higher lands which surround the Colorado 
desert, as well as those which lie in and about an extensive territory bordering the Colorado 
on either side, are marked by these dry water-courses, which have received from those who 
are most familiar with this region the technical name of ‘‘washes;’’ in Spanish they are called 
‘‘arroyos.’’ They form a striking feature in the topography of the country, and are generally 
