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GEOLOGY OF THE SIERRA NEVADA. 45 
between the Merced and the Stanislaus rivers, but it is divided into two 
portions along its whole extent by the belt of argillaceous slates, already 
mentioned as containing Lima LErringtouw and other Jurassic fossils. This 
clay slate belt is also interesting as being intimately associated with the 
Great Quartz Vein, which is so prominent a feature of the geology of this 
region. 
There is a depression near the centre of the metamorphic belt just de- 
scribed, in which the Petia Blanca and Moccassin and Woods’ Creeks run, — 
the first-named a branch of the Merced, the two latter of the Tuolumne 
River. This depression is occupied by a continuous belt of argillaceous 
slates, having a width varying from half a mile to a mile or more. These 
slates are very finely lammated, homogeneous in character, of a dark-gray 
color, which is of a light rusty-brown at the surface. At a sufficient depth 
to be quite beyond the reach of atmospheric influences, this rock becomes 
almost black. The north edge of this belt passes, almost in a straight line, — 
although with minor flexures,— from the Pine Tree Mine, on the Mariposa 
Estate, just south of Coulterville, and about one fourth of a mile northeast 
of Jacksonville. On approaching the Stanislaus River it widens a good deal, 
and becomes more irregular in its outline. Along a considerable portion of 
its course the argillaceous slate belt is accompanied by a band of serpentine, 
which is more than a mile wide just beyond the Benton Mills, but gradually 
narrows down to a few yards, and disappears just beyond the Mary Harrison 
Mine. It then comes in again near Coulterville, and continues, with a width 
of from a quarter to a half a mile, to the north fork of Moccassin Creek. It 
again makes its appearance between Grizzly Gulch and Kanaka Creek, and 
here it is accompanied, on its northwestern edge, by a thin band of porphyry. 
All the outcrops of serpentine thus far mentioned are on the northeast edge 
of the argillite belt; but farther to the northwest, towards the Stanislaus, 
this rock widens out into patches of irregular form, having quite an exten- 
sive area, and occurring on both sides of the slates. 
Associated with the serpentine is the very remarkable mass of quartz, 
known as the “ Great Quartz Vein,’ which presents many features worthy of 
careful study, besides being important from the mines which have been 
worked at so many points, either in the mass of the vein itself, or in its im- 
mediate vicinity. This great lode may be traced from the Mariposa Estate 
to near the centre of Amador County, a distance of fully eighty miles. It is 
not visible on the surface at all points, but, like the limestone belt already 
