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GRAVEL AND VOLCANIC FORMATIONS: SOUTHERN COUNTIES. 125 
phy’s, the strike of the limestone is N. 80° E,, and its dip is to the southeast 
at an angle of 75° or 80°. The outcrop of this rock at Murphy’s, where the 
belt is a little over a mile wide, is in part similar to that described as occur- 
ring on the other side of the Stanislaus, and the cavities have been exten- 
sively washed for gold. On the northern side of the town, however, the 
limestone rises in high hills, very conspicuous from their peculiar blue- 
gray color, so different from the usual tinge of the bed-rock in the mining 
districts. This character of the limestone continues between Murphy’s and 
San Domingo Creek, on the trail to Cave City. Just beyond the last-named 
creek it disappears again, and is not seen until we arrive near Cave City, 
about five miles, in a direct line, northwest of Murphy’s. At this point the 
limestone belt is very conspicuously exposed, resembling in the character 
of its outcrop the hills north of Murphy’s. The strike of the beds at Cave 
City is about N. 30° W., and the dip to the southeast at an angle of 60°, the 
width of the belt being here about one fourth of a mile. From this point 
the limestone can be seen trending to the northwest and presenting, for 
a distance of two miles, a series of precipitous and almost bare outcrops. 
Beyond this it has not been seen by any member of the Geological Survey 
Corps until the neighborhood of Volcano is reached (see page 87). 
The bed-rock on the northeast side of the limestone belt in Calaveras 
County is mostly slate of a very quartzose character, or quartzite. It is 
much less auriferous than the rock on the other side of the limestone, there 
being hardly any mining, whether quartz or placer, to the northeast of that 
belt of rock. 
§ 2. Gravel and Volcanic Formations in Calaveras, Tuolumne, and Mariposa Counties. 
The gravels are comparatively of so little importance in the region south 
of the Mokelumne, that in a rapid description of the geology it is not desira- 
ble to describe them separately from the volcanic formations. The latter 
are, however, of great extent and interest; but the region requires a much 
more detailed investigation than has yet been given to it, based on an accu- 
rate map; and this field work would have to be followed by a microscopic 
examination of the specimens collected. All that can be done, at present, is 
to point out roughly the extent of the region covered by the volcanic beds, 
and to give the character of the gravels at the principal points where they 
have been worked. 
