Geology of the Mother Lode gold belt. — Fairbanks. 2 IT 
posited, and thus cause a banded appearance. This is exemplified 
in vein quartz, which shows a slight banded structure even when 
massive, and in the division of most of the great quartz deposits 
into layers more or less thick, with talcose material between them. 
To account for the phenomena, I wish to advance an entirely dif- 
ferent theory, against which I do not see that any valid objection 
can be raised. It is this : That those portions of the lode so 
enormo.usly expanded are simply coarse, basic dikes, of no great 
regularity or continuity, which, lying in the course of the fissure, 
have been acted upon in a peculiar way by the penetrating liquids 
and gases. These, through metasomatic processes, have removed 
part of the original constituents and substituted others. A strong 
confirmation of this theory is found in a large body of unquestion- 
ably eruptive rock, near Jamestown, Tuolumne county, and about 
half a mile from the Mother Lode. It has ver} r mucji the same 
appearance as the vein matter of the lode, except that there is no 
mariposite. It is seamed with small veins of quartz and in surface 
decay produces the same red oxide of iron. The only real differ- 
ence is that the process of substitution is not so complete as in the 
Mother Lode. Dikes that have undergone a partial change often 
occur penetrating the Mother Lode vein matter, and at times 
they are slightly impregnated with mariposite. 
An additional reason for adopting this theory is found in the 
sudden and great expansion and contraction of the lode, as on 
Moccasin creek, where it widens from only a gouge seam to fifty 
feet in the course of a few rods. In a small vein, where the ex- 
pansion and contraction are only a few yards apart, the variation 
in width can easily be accounted for by a movement of the walls 
sufficient to bring two prominences or two hollows together. How- 
ever, this cannot be possible in the case of the Mother Lode where 
they are sometimes separated by only a gouge for a mile or more 
and then for a distance of half a mile spread to a width of several 
hundred feet. 
Although it is rare to find any mariposite or ankerite where ser- 
pentine does not form one wall, yet there is an exception in the 
case of a long barren vein, which extends north from Oregon bar 
ten miles into Placer county. It has a width of forty feet, and 
contains these two minerals in large amount. 
The veins of the lode dip, almost invariably a few degrees less 
than the inclosing rocks, and it is usual to find the foot wall rocks. 
