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 Erringloni 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 Peiia 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 laminated, 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 annlhte 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 



