Mineral Deposits of Eastern California. — Fairbanks. 155 
with silica along narrow cracks. The clean quartz usually 
forming the vein I cannot account for in any other way than 
by filling of cavities, as it does not seem possible that a 
replacement of the ferro-magnesian silicates and other 
minerals could occur without leaving chloritic stains or other 
signs in the resulting mass." It seems to the writer that the 
evidences are opposed to Mr. Lingren's views of the presence 
generally of open fissures, during the formation of the quartz. 
Veins which are known to have been formed in open fissures 
almost invariably show a more or less pronounced banded 
structure, caused by successive additions through a long 
course of time during which the conditions did not remain 
absolutely uniform. In an open fissure the deposition cannot be 
conceived as having taken place except from the wall inward, 
leaving frequently a cavity in the centre. The best examples 
of veins which the writer recognizes as having been formed 
in open fissures are those at Bodie, and at Sulphur creek in the 
Coast ranges. At Bodie there is in addition to a very regu- 
lar banding a pronounced comb structure in places. As 
far as the writers acquaintance goes with the gold veins of 
California this sort of banding as well as comb structure is 
very rare. Emmons says:* "The comb structure of veins on 
which the early geologists founded their theory that a vein 
was necessarily the filling of a pre-existing open cavity, is of 
comparatively rare occurrence." It is not denied that cavities 
were abundant, for the presence of empty spaces lined with 
crystals is evidence of that, but that is far from demonstrat- 
ing the presence of open fissures. The removal of material 
from the crushed zone by the mineral solutions may not 
always have been accompanied by substitution at all points, 
owing to unfavorable physical or chemical conditions, thus 
giving rise to local chambers lined with drusy crystals. 
The ribbon-like quartz so frequent along the Mother lode 
shows indeed a banded struct lire, as its name indicates, but 
of a different kind. It may have been formed by successive 
additions to tin- vein on the outside, the streaks or layers of 
chloritic matter indicating successive tightening of the walls. 
alternating with their loosening and removal and the accom- 
panying more rapid accumulation of quartz. Emmonsf sug 
*Am. Inst, of Min. Eng., 188G, p. 133. 
tAm. Inst. Min. Eng., vol. win. 
