CHARACTER OF IGNEOUS ROCK—SODA LAKE. 
101 
range runs north and south, forming with the others a series of triangles, enclosing valleys, 
or rather playas, between the hills. In its course eastward the river either canons through or 
passes round these ranges to reach the lower level; hence its occasional deviation to the west¬ 
ward. 
The intruding rock forming the axis of these isolated ranges and lone hills is chiefly amyg¬ 
daloid, a reddish felspathic rock, approaching trachyte in texture, the cavities filled with 
chalcedonic geodes. Masses of pure felspar rock occur occasionally. Greenstone and greenstone 
porphyry are the chief varieties of volcanic rock. Jasper and compact quartz rock, of various 
hues, lie in contact with*the foregoing. These rocks, flanked by the conglomerates, constitute 
the bulk of the hills of the Mojave valley. 
At the third camp on the Mojave, sixty-six miles down the river from the first crossing, the 
amygdaloid rock rises abruptly from the plain beside the river on its south side ; at some dis¬ 
tance on the north side it rises up again. In their elevation they have uplifted a stratified 
sandstone which, in some places, is converted into a compact quartz rock ; in others, it puts on 
a jaspery and opal appearance in the fractures, while at a distance it preserves its laminated and 
sandy texture. The amygdaloid is reddish, but less pyritiferous than usually met with. A 
great variety in the appearance of the volcanic rock prevails over this district, being sometimes 
highly cellular, the cavities filled with chalcedony ; again it is a compact rock, with defined 
quartz crystals interspersed—in some places pyritiferous, in others not—the felspar generally 
approaching a red brick color ; in a few instances becoming grayish, and resembling trachyte. 
A yellowish green clay forms on the surface of some of these volcanic hills, arising from the 
decay of the iron and copper pyrites present. This tint is sometimes communicated to the soil 
in such a quantity as to render it visible at a distance of several miles. 
Dykes of a green felspar porphyry cut through the amygdaloid in some places ; one well 
marked instance is on a hill north of this (3d) camp. The amygdaloid is fissured in several 
places, and the fissures filled with seams of carbonate and sulphate of lime ; these seams run in 
the same direction with the upheaval of the hill—that is, east and west. The carbonate is crys¬ 
tallized in the rhomboidal form. North from this the ground rises with a gentle slope, thence 
norcheast. The highest hills of the neighborhood are in that direction ; they have the same 
porphyritic outline as the hills close by, and run 50 or 60 miles northward, constituting a well 
marked chain of hills, whose southern prolongation, about 10 or 12 miles from here, is a mass 
of porphyritic hills heaped together. 
Seventy-one miles down the river (from the first crossing) a range of granitoid and porphyry 
hills cross the course of the stream, and through which the river canons ; loose fragments of 
serpentine and epidote were scattered about, with jaspers, chabasite, red and yellow porphyry. 
A bed of unconformable conglomerate lies in contact with the axis rock; it is 60 to 80 feet 
thick, and the pebbles made up of jasper, porphyry, and epidote, with a fine paste of sand and 
clay, the fragmentary scattered pebbles derived from this conglomerate being denuded. 
This chain runs northwest and southeast, as do many of the lesser ranges, but it cannot be 
spoken with certainty of many of them ; the district being so disturbed, some of the low ranges 
running north and south, and inclosing triangular valleys, as before described. 
The same stratified sandstone which caps the eastern flank of the sierra to the Mojave river 
is also found here, and inclined also at a small angle-to the east, (4°,) and sloping eastward for 
12 miles, when it terminates in Soda lake. 
Soda lake is a flat, dry lake bed, or playa, about 20 miles long and 12 broad, of an elliptical 
