21 G The American Geologist. April, i89i 
A white crystalline mineral resembling dolomite is associated 
with the mariposite. It consists of the carbonates of calcium, 
magnesium, and iron in varying proportions. The iron is gener- 
ally present in large amounts, forming that variety of dolomite 
called ankerite, and giving rise to the red oxide so abundant in the 
surface decay. The iron at times may nearly or quite disappear, 
forming dolomite, or the calcite may be so nearly lacking that it 
becomes magnesite. These minerals form the great mass of the 
vein matter at points where the lode is so enormously expanded, 
as at the Josephine mine, the vicinity of Coulterville, Pinon 
Blanco, Quartz mountain, Whisky hill, Raw Hide mine, Carson 
hill, and Chili creek. 
The most interesting fact connected with these minerals as they 
occur in the vein, is their relation to the character of the inclosing 
walls. It is very rare that any vein matter is associated with the 
quartz when the walls of the fissure are slate, diorite or diabase, 
but it is almost always present when one wall is serpentine, or 
when serpentine lies only a short distance away ; hence, it is 
natural to believe that there is some relation between the walls and 
the vein matter. A basic rock, such as that must have been from 
which the serpentine was derived, undergoing decomposition, 
would afford opportunity for the liquids circulating through the 
fissure to abstract such bases as are found in mariposite and the 
dolomite or ankerite, and under the proper conditions to deposit 
them. This was my belief during a portion of the field work, but 
upon further study, in the laboratory, doubts began to arise as to 
the possibility of these minerals having been formed in that man- 
ner, — whether such immense fissures as those demanded by the 
amount of vein matter could have existed. If it be urged that the 
space between the walls need not have been so large at any one 
time, and during a gradual opening the filling kept pace, then, 
where are the signs shown in the structure of the deposit ? The 
vein matter is absolutely massive ; there is no trace of a banded 
or bedded structure, and it would seem necessary that such a 
structure should exist, to a greater or less degree, in deposits on 
the walls of a fissure, either by reason of the successive additions, 
in which it would hardly be possible that the currents would be 
uniform, or the conditions the same through protracted intervals ; 
or bj T successive openings and closings of the fissure, in which case 
more or less of the wall material would adhere to the matter de 
