290 
GEOLOGY, 
LIMONITE. 
Burns’ Creek , Mariposa County .—An outcrop of hydrous sesquioxide of iron occurs near the 
hanks of this creek, on the right of the road going south. Its position, and its geological asso¬ 
ciation, has already been noted in Chapter II. 
It is associated with a quartz vein, and both together form a bed about twenty-five feet thick, 
lying conformably with talcose and chloritic slates. The layers of slate adjoining the vein are 
more or less charged with the oxide, and some thin layers are distinctly intercalated. The ore 
lies upon the surface in great solid blocks, from two to four feet in diameter. It is compact, of 
a dark-brown color, and breaks with a smooth, conchoidal fracture. Its powder is a dark rusty- 
brown, and its hardness about five on the scale of Mohs. It is peculiarly compact and hard, and 
is unlike any specimen of similar chemical composition that I have seen. 
Its position and peculiarities indicate that it has resulted from the decomposition of sulphuret 
of iron, (iron pyrites,) and that it now forms the “gossan” of a vein of sulphuret below the 
surface. This is probably the case, hut the mass does not present that cavernous, porous, and 
friable condition in which gossan is usually found. An examination by the blowpipe did not 
show the presence of any sulphur. It is easily reduced, and when carbonized fuses to a mag¬ 
netic globule. At some future day this ore may he valuable for making iron, hut at pfesentthe 
scarcity of fuel and the cost of labor will not admit of its being profitably worked. It contains 
about fifty-six per cent, of iron. 
COPPER PYRITES. 
Great Basin .—Yellow copper pyrites was met with on the slope of the Great Basin, about 
seven and a half miles east of Johnson’s River, in the form of loose fragments, covered with a 
slight green coating of the carbonate. I followed up the bed of a dry brook, along which the 
fragments had been transported, until I found the parent vein. This is in a micaceous granite, 
forming one of a series of small isolated hills or ridges at the upper part of the slope, and not 
more than three miles from the foot of the main range. 
The trend of the vein was ascertained to he nearly north 75° east, (magnetic,) and its 
inclination to the north ; hut it is nearly vertical. The outcrop consists of quartzose rocks, 
containing masses and coatings of carbonate of copper and the fine oxide of iron, which some¬ 
times results from the decomposition of pyrites. These indications of a vein were spread over a 
width of twenty to thirty feet, and were traced for one quarter of a mile. This is a valuable 
vein, and I have no doubt will show some splendid ore on being opened. It is possible that it 
is auriferous ; hut I had no means of determining this question. Its position is such as to 
discourage any attempt to work it at present. There is no water within three or four miles of 
the place. Timber can he had from the mountains, four or five miles distant. The ore could 
he transported to Los Angeles, by mules or wagons, a distance of nearly eighty miles, by the 
way of Williamson’s Pass. 
VITREOUS COPPER. 
Williamson’s Pass .—A sulphuret of copper occurs about seven miles below the summit-level 
of Williamson’s Pass, on the north of the valley and on the slope of a granite hill, about ninety 
