124 THE AURIFEROUS GRAVELS OF THE SIERRA NEVADA. 
the limestone belt acquires a width at Columbia, and from there west to the 
Stanislaus, of from one and a half to two miles. Great numbers of dykes of 
dioritic rock cross it nearly at right-angles to its strike; they vary in thick- 
ness from a few inches to many feet. It would appear that these dykes are 
a peculiar feature of the limestone belts, since they occur not unfrequently 
in this connection.* It is not easy to see why there should be any special 
reason for their occurrence in the limestone belt in preference to other por- 
tions of the bed-rock series. It may be that they are more conspicuous in 
that position, on account of the marked difference in color of the two rocks 
and the complete exposure of the surface of the limestone. This rock, which 
between Kincaid Flat and the Stanislaus was formerly covered by a heavy 
mass of detritus, has been washed clean, and exhibits a most curious appear- 
ance, as already noticed in describing the bed-rock in the vicinity of Volcano. 
Its surface is everywhere worn into cavities, sometimes as much as fifty feet 
deep, but oftener ten to twenty, the inclination and direction of which coin- 
cide with the dip and strike of the rock itself, while their sides are smooth 
in appearance, looking as if the material had been removed by some cor- 
roding, rather than eroding, agency. ‘These cavities were once filled with 
auriferous materials, which also appear to have extended over the whole 
surface in distinctly stratified beds, rising several feet above the general 
level of the higher projecting portions of the limestone. 
Beyond the Stanislaus the limestone belt is quite irregular in its develop- 
ment; but it is, as before remarked, a prominent feature in the bed-rock 
geology of Calaveras County. At Abby’s Ferry, where it is intersected by 
the river, it is more than a mile wide, and rises in highly picturesque cliffs 
on either side. The strike of the limestone here is nearly east and west, 
magnetic, and the rock underlying it is almost exclusively made up of 
hornblende, which occurs in very coarsely crystallized masses. On the west 
side of the Stanislaus, away from the immediate vicinity of the river, the 
limestone is covered by the lava flow of Table Mountain; but it is well 
exposed again in the neighborhood of Douglass Flat and Murphy’s. The 
strike of the beds in this county is very variable. At the Ferry, as already 
remarked, it is nearly east and west; and while the general trend of the 
formation from Vallecito to Murphy’s is about north and south, yet the 
strata appear to run in a direction nearly at right-angles to this. At 
the Blue Wing Mill, for instance, three fourths of a mile north of Mur- 
* See ante, p. 87. 
