GEOLOGICAL REPORT. 335 



highly altered stratified rocks. Still farther on, a prominent white mountain south of 

 the road was found to consist of purely white subciyBtalline and finely crystalline 



dolomite, evidently altered from a dark-gray ma-nesian limestone whirl, still forms 

 part of the mountain. The contact between the two modifications exhibits no straight 

 line, hut follows irregularly secondary fissures. The stratification is obliterated by 

 the metamorphosis. Xo fossils were noticed. Close by some slates crop out, and a 

 dike of a greenish decomposed igneous rock. 



Gibraltar ( Veok furnishes a striking example of the repeated sinking and re-ap- 

 pearing of the water, modified in its qualities by the strata which it percolates. At 

 our camping-place in the Middle (late the water was insufficient, ami tasted disagree- 



it would be easy not only to secure a permanent supply at that (iate, but that a much 



eral miles lower down, ami that thus the long waterless distance to ( 'arson Lake could 

 be much shortened. 



The Black Mountains form only a comparatively low ridge east of Carson Lake, 



sions, probably of later date. On the west side, dark-colored vesicular rocks were 

 found in considerable quantity, and above them altered clay slate, gneiss, and com- 

 pact quartz forming a vein or stratum. Farther north the mountains have a stratified 



gray, red, scoriated, vesicular, &c. 



Alkali Valley, formerly a branch of Carson Lake, is still mostly a miry salt Hat. 

 with a great deal of loose drift-sand on the surrounding beach and benches, especially 

 on the east side. 



Carson Lake, as far as the bend of Carson River. In that district rocks prevail similar 

 to those of the Black Mountains. Near the lake we find scoriaceous vesicular out- 

 crops of dark gray and red color and igneous origin, and, lining the hills, a great deal 

 of calcareous tufa, in places enveloping numerous particles of the red rock, and then 

 readily mistaken as such; also, considerable of a sedimentary rock of white color, 

 mostly composed of pumice and other igneous material, and allied to the volcanic 

 tufas. Rocks of the basalt or greenstone group, partly vesicular, were also observed 

 at various points between the fake and the bend of Carson River, and along Walker 

 River, to the exclusion of other igneous rocks, except some in the main divide between 

 the two rivers, which are distantly related to the trachytic porphyries.* One speci- 

 men of the latter has a loose porous texture, and contains in the light-gray feldspathic 

 matrix crystals of glassy ( !) feldspar and brown mica. Others contain hornblende 

 instead of the mica, especially higher up on Carson River, and appear more allied to 

 those from Weber River. 



* I do not think that there are any rocks along our line of survey which can properly be called basalt. Several 

 of the basaltic rocks in the neighborhood of Carson River and Carson Lake resemble more the lavas from Monte 

 Somma and Vesuvius, one especially a lava which ernpted as, late as 1806, and prest 

 which small green crystallizations can be recognized only when the rock has begun to decompose. 



