308 
GEOLOGY. 
was observed at Benicia (Navy Point) and Ocoya creek. At the antimony vein, near the Pass 
of San Amedio, it occurs abundantly in crystals, similar in their size and 
modifications to that represented in the figure. They are found in the 
decomposed portions of the vein, and good cabinet specimens can be 
obtained. The mineral was also observed in the Great Basin ; near the 
entrance to the New Pass, and at Carrizo Creek near the Desert. It was not found at either of 
these places in quantities sufficient for the purposes of the arts or agriculture. 
CALCITE—CARBONATE OF LIME. 
Several small masses of transparent calcite, equal to the variety called Iceland spar , 
were picked up on the surface of the Colorado Desert, north of the Emigrant Road, from the 
mouth of the Gila to Carrizo Creek. They are probably washed out of the soft Tertiary strata, 
in which so many concretions or clay-stones occur. One of the specimens is a perfect crystal, 
an unmodified rhombohedron—angle 105°. It is about three-quarters of an inch in diameter, 
and its surface is slightly dimmed or ground by the action of the driving sand. 
In the pass through the Peninsula Mountains, called Jacum, several miles south of Warner’s 
Pass, thin seams of crystallized carbonate of lime are abundant. The crystals are acute 
rhombohedrons, about half an inch long, and are thickly implanted upon a thin crust of the 
same mineral, so that they’Torm interesting cabinet specimens. Small crystals of similar form 
were noticed in veins and cavities of the white limestone in the Tejon Pass. 
At the quicksilver mine, New Almaden, this mineral occurs in obtuse rhombohedrons, lining 
the cavities in the veins. Veins of calcite are very abundant in this mine, and they traverse 
the ore in all directions, cutting directly across the small veins and strings of the cinnabar, so 
that their continuity is completely broken. Small globules of bitumen are sometimes found 
implanted among the crystals. 
MAGNESITE—CARBONATE OF MAGNESIA. 
Immense beds of nearly pure carbonate of magnesia were found in the foot-hills of the Sierra 
Nevada, between the Four Creeks and Moore’s Creek. These beds are from one to six feet in 
thickness, and are interstratified with talcose and chloritic slates and serpentine. The mineral 
is snowy white, and being on the side of a high ridge, the outcrops are visible from the plain 
for a distance of several miles. 
The rock is compact and homogeneous, and does not show any traces of crystalization, but 
breaks with a smooth, conchoidal surface, somewhat resembling broken porcelain, but it is not 
so translucent. Its hardness is not less than 5.2 of the scale of Mohs. It is so hard and dense 
that it cannot be cut with a knife, but maybe ground perfectly smooth, and polished. No other 
minerals or impurities were observed with these beds ; they appear to be nearly pure. An 
examination showed the presence of a little silica and oxide of iron, probably a carbonate, but 
of the latter there is not enough to affect the color of the mass, or stain it by decomposition. It 
contains about forty-seven per cent, of magnesia and fifty-two of carbonic acid. The following 
analysis, by Professor T. S. Hunt, is of the magnesite of Sutton, Canada East, where it occurs 
in large quantities I 
Carbonate of magnesia..83.35 
Carbonate of iron....... 9.02 
Silica, (mixed).... 8.03 
1 Report of the progress of the Geological Survey of the Canadas, by Sir W. E. Logan, 1848. 
