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 caiions 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 thi.s the ground rises with a gentle slope, thence 

 northeast. 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 caiions ; 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, wlien 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 



