WESTERN EDGE OF THE DESERT. 
121 
the upheaval, the fractured gneiss bed became imbedded in the yet pasty magma of the granite. 
The chemical character of the granite changes from west to east as it crosses, so that the pre¬ 
dominating mineral apparent in it is as follows : 
West.—1. Granite, (micaceous;) 2. Gneiss; 3. Syenite and hornblendic rocks, with 
protogine ; 4. Felspathic porphyry ; 5. Trachyte.—Bast 
The eastern flank of the sierra is lined by a range of volcanic hills, made up at the base of 
reddish felspar, porphyry, and trachyte, upon which is superimposed a rough conglomerate of 
primary pebbles. 
With our present knowledge it may be difficult to fix the exact geological age of the sierra. 
As the Miocene tertiaries of San Diego rest unconformably upon the granite west of San Pas- 
quale, the elevation of the great mass of the granitic axis is post-Miocene; but during, and 
perhaps before, that period a submarine and partially elevated axis was already in existence, at 
which time was probably formed the terraced summits in the valley of Warner’s ranch. Since 
then the later granites and porphyries forced their way through the eastern sides of the chain, 
and may have contributed to raise the whole range very much higher ; yet, between that eleva¬ 
tion and its present height, an interval of considerable time occurred to produce the unconsoli¬ 
dated sands and clays of the desert; and as this last elevation of the basin has been uniform 
over its whole breadth, (100 miles at this point,) such an upheaval could not have occurred 
without also lifting the whole mass of the sierra to some additional higher level. There are 
thus three periods of elevation of these mountains, the latter two of which are post-Tertiary, 
(post-Miocene.) 
Carrizo creek runs over a series of stratified clays and gravels, derived from the decomposition 
of the primary rocks, chiefly syenite, loose drifted pebbles of which cover up the sand beds 
of the valley. Through this sand the Carrizo, in places, cuts its way very deeply. At the 
camp (June 3) on the river, the sand is deposited unconformable to the primitive rocks, upon 
whose side it reposes. It is mainly composed of disintegrated syenitic rock. 
Having travelled seven miles down the Carrizo, from the point where it is first struck on the 
descent, the trail leaves the river and rises a terrace of stratified clay gravel, with interlaced 
layers of gypsum. 
This is the rim or margin of the desert, and constitutes the edge of a terrace, the uppermost 
of two. The level of this terrace is 430 feet above the sea, and slopes gently south and east, 
until it meets with the beds of fine clay constituting the desert bottom. Coarse gravel pebbles 
are strewed plentifully along the trail, with volcanic debris, broken fragments of basalt, por¬ 
phyry, purple felspathic rock, and loose blocks of syenite ; through this the creek cuts its way. 
This heavy deposit of sand is denuded in places by pluvial and fluviatile action, posterior to its 
deposit, and before its elevation above the sea, the wearing of the porphyritic and igneous 
rocks occurred, whose pebbles are loose, upon its surface, and which cannot be classed with 
results of causes at present at work. Between the river and the volcanic rocks at the east, the 
stratification of the sands are well marked; and while they are unconformable, they have a 
slightly arched line of deposit, giving an apparent dip east and west of the river, which may 
be due to its having been deposited in an estuary mouth, or in the manner in which tidal bars 
are formed. 
The storekeeper inhabiting the adobe house, newly built at camp, informed us that, for the 
eight months previous to our visit, it had not rained but once, and then for eight hours heavily ; 
16 U 
