212 The American Geologist. April, i89i 
the schists and slates abut against it for a distance of twenty 
miles, and in the vicinity of the eruptive mass they are greatly 
broken and metamorphosed. 
The region traversed by the Mother Lode is one characterized 
by vertical or steeply inclined rocks which are either eruptive 
dikes or sedimentary strata. Each of the four formations, granite, 
slate, serpentine, and diabase is characterized by a different sur- 
face and soil. The two formations to which the most important 
topographical features owe their existence are the long, narrow 
bands of slate and the adjoining diabase dikes. The uniform con- 
formability of these great dikes to the stratification of the sedimen- 
tary rocks, their hard and indestructible character, and their 
juxtaposition with the soft, easily eroded slates, have given rise to 
those long, deep and narrow canons leading down to the main 
rivers that cross the lode. 
In Mariposa county, only, does the diabase appear in any great 
amount on the east side of the lode ; there it forms the Mount 
Bullion range, which rises four thousand two hundred and fifty 
feet. Bear mountain, of Mariposa county, and its continuation 
north in the high ridge west of moccasin creek ; the Bear mountain 
range of Calaveras county ; the low ridge through Amador county ; 
and the high hills west of the north fork of the Cosmunes river in 
El Dorado count}" are the prominent features of this rock west of 
the lode. This diabase forms a continuous dike west of the black 
slates through Mariposa county. In Tuolumne county it is broken 
for a short distance, and near the northern border of the county it 
appears three miles west of the lode. This is due to a longitud- 
inal compression, the effect of which is also seen in the slates 
which bend nearly as much in the same direction and away from 
the course of the lode which pursues nevertheless a nearly direct 
though less regular line. The serpentine also partakes of the 
westward deflection, but its massive character indicates that this 
peculiar position was assumed at the time of the outburst and was 
not induced by the subsequent folding, for in the latter case it 
would have been rendered fissile. This is interesting as it gives 
a clue to the sequence of events in the history of these rocks. 
Near the northern edge of Calaveras count}' the black slate is 
again found in the course of the lode, and it is not only bordered 
on the west by the diabase but the latter forms many parallel 
dikes in it. The quartz ledges usually appear at the contact of 
these two formations. 
