110 
GEOLOGICAL REPORT—THIRTY-FIFTH PARALLEL. 
Ho. 46. Leroux's spring. —This is a porphyitic lava, crystals of glassy feldspar being dissem¬ 
inated in a compact dark base. Distinct crystals of black hornblende are abundant. The speci¬ 
men is very small, and much weathered. 
No. 47. From a lava stream near Leroux’s spring. —According to Mr. Marcou this specimen 
is from a stream of basaltic lava from San Francisco mountain. It very much resembles No. 
45, having a similar color, composition, and aggregation. In this specimen the vitreous grains 
are more distinct, larger, and better characterized. The color is olive-green ; they are easily 
pulverized, and are infusible before the blow-pipe. They are distinct from quartz, and are 
probably olivine. 
The weathered surface of this rock has a rusty brown color, but the discoloration is only on 
the surface, or below it for an eighth of an inch ; the rock is slightly bleached. 
No. 48. Volcano of San Francisco mountain. —This is a light amygdaloidal lava, full of elon¬ 
gated air-cells, from the size of a pin’s bead and smaller to half an inch in length. These cells 
are very smooth on the interior, and are of a lighter color than the surrounding portions. The 
cells are all elongated and flattened in one direction, showing the mass to have been in a state 
of tension when in a fluid or pasty condition. The outside or weathered surface has a reddish 
color. The color of the rock is an ash-grey with a faint shade of green. 
No. 49. San Francisco mountain. —This is a fine specimen of volcanic scoria, or porous slag. 
It is an irregular porous mass, very light, and apparently formed by the dripping of the froth 
and scum from the surface of fluid lava. The outer surface is smooth, and presents several 
shades of color—black, purple, drab, and grey. It resembles scoriaceous specimens from the 
crater of Ivilauea and other volcanoes. 
No. 50. Aquarius mountains. —This is a beautiful porphyry. The feldspar is in distinct, 
well-formed glassy crystals, from one-eighth to three-eighths of an inch long. The base is com¬ 
pact, and of a light purple or rose-color. 
No. 51. From the base of San Francisco mountain , near the Colorado Chiquito. — This specimen 
is a beautiful amygdaloidal lava, with the cavities very similar in their size and form. They 
are lined with a white zeolitic crust, without crystals, and in such small quantity that the 
species cannot he readily determined. The base of the rock is dark, compact, and hard, and 
presents a pleasing contrast with the white cells. I find in this rock olive-green grains similar 
to those which were found in Nos. 45 and 47 ; and, in addition, some which are red, and more 
nearly resemble garnet than olivine. On trial before the blow-pipe, however, they are found 
to he infusible ; and, as the other characters are exactly those of the green variety, I am led to 
regard them as identical. 
No. 52. Near the Picacho. —This specimen appears to have been a loose boulder, as rounded 
and water-worn surfaces are found on the specimen. It is an exceedingly hard and compact 
black rock, breaking with a conchoidal fracture, very brittle, and without any trace of crystal¬ 
lization. The weathered surface is deeply wrinkled, and marked with little pits, looking as if 
it had been soft and undergone slight contraction when cooling. In this peculiarity it resem¬ 
bles many of the similar black basaltic pebbles found by me along the Colorado and Gila rivers, 
and on the desert, near Carrizo creek, California. 
No. 53. Camp near Picacho. —This rock is similar to No. 52, and differs only in being more 
compact and vitreous, being not unlike obsidian. Mr. Marcou has ticketed this “Basalte 
obsidienne.” It contains one or two isolated, transparent, and colorless vitreous grains, which 
resemble quartz, but are softer and mor.e easily crumbled. This specimen, like the preceding, 
appears to have been taken from a transported mass. 
No. 54. Third chain beyond the Colorado. —This is a small specimen, and much altered by 
weathering. It consists of a light-colored feldspathic base, porphyritic, and containing 
isolated grains of quartz. It may be called a quartz porphyry. 
No. 55. Mojave river. —This is a beautiful green-stone porphyry, the base being a compact, 
dark green trappean rock, bearing isolated crystals of a white or light yellow mineral, having 
