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. — East 



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 iiction, 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 apparer^t 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 



