GRAVEL AND VOLCANIC FORMATIONS : TUOLUMNE COUNTY. 



141 



being hardened into a solid mass. The andesite is of a dark gray color, and 

 contains long, prismatic crystals of black hornblende. As is often the case, 

 the masses of which the breccia is made up grow larger toward the top of 

 the deposit, and there attain sometimes a diameter of three feet or more. 

 The unequal wearing away of the cementing material often causes the vol- 

 canic mass to assume a very rough and irregular surface. Over the volcanic 

 table-land in the vicinity of Soulsby ville are scattered large blocks of a more 

 compact and finer grained andesite than that occurring in the breccia be- 

 neath It is of a grayish-brown color, and contains minute crystals of feld- 

 spar. These fragments appear to be the remnants of a bed of lava which 

 was once continuous over a considerable area of the surface in this region. 

 The gravels under the volcanic deposits in this vicinity do not seem ever to 

 have been extensively worked for gold ; they appear to be shallow and irregu- 

 lar, but the surface is so covered by the lava, that but little can be ascer- 

 tained in regard to the real position of the channel. For an illustration of 

 the character of the table-land near Soulsbyville reference may be made to 

 the section given on Plate E, Fig. 3. 



Volcanic rocks also occur over a considerable surface between Soulsbyville 

 and the Bower Cave; they are andesitic in character and are associated with 

 gravels ; but the latter do not appear to contain a paying amount of gold • at 

 least, although they seemed to have been prospected in many places they 

 have not been worked to any extent. 



Farther south in Mariposa County there have been, in former times very 

 productive placer diggings, but all shallow; and these have been long since 

 worked out or abandoned to the Chinese. The shallowness of the gravel in 

 this region lias been the chief reason why water has not been brought down 

 from the higher parts of the Sierra, for which undertaking long and costly 

 ditches would have been necessary. 



As we proceed south from Mariposa County we find the gravels becoming 

 less and less productive and important, Gold has been found in some quan- 

 tity in some of the gulches in Fresno County, and the detrital accumulations 

 in the beds of the lower tributaries of the Chowchilla, San Joaquin, and 

 Fresno rivers have been worked to some extent ; but we have no particulars 

 of importance in regard to them. Their production has been so small that, 

 so far as known to the writer, the limits of the gold placer mining region of 



the Sierra may be said to terminate with the southern limit of Mariposa 

 County. 



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