40 H. W. Fairbanks — Tin Deposits in California. 



Carboniferous, but further than that we have no information. 

 Extensive bodies of porphyry also border the granite. They 

 vary much in appearance, in some places being of a grayish 

 color and almost devoid of distinct crystals, but more generally 

 the rock presents a groundmass almost black in color in which 

 are sprinkled white feldspar crystals and more rarely those of 

 quartz. It is difficult to say which is the older, the granite or 

 the porphyry. In the neighborhood of the junction both 

 rocks change their character somewhat, although each is dis- 

 tinct. With perhaps one exception, none of the fine-grained 

 granitic dikes abundant in the coarse granite were noticed 

 penetrating the porphyry, but there are bunches and dike-like 

 protrusions of the latter in the granite near the contact. 



The granite is distinctly intrusive in the metamorphic rocks, 

 as bunches of it project up through them here and there. The 

 granite varies considerably in texture although the main body 

 has a uniform composition. Macroscopically it shows two 

 kinds of feldspar, one a pale brownish color and the other 

 white, abundant quartz and a small amount of the dark sili- 

 cate. Under the microscope it is seen to consist of plagioclase, 

 orthoclase, and quartz in nearly equal proportions, with a much 

 less amount of biotite mica. Towards the outer edges of the 

 granitic boss in which the mine is situated are numerous dikes 

 of a very fine-grained granite consisting almost wholly of 

 quartz and orthoclase in interlocking grains. The proportion 

 of quartz seems to be greater in some of these dikes than in 

 the coarse granite to which they are genetically related. 



The Vein System and Tin IJeposits. — The semicircular area 

 of granite and portions of the adjoining porphyry have been 

 fissured in a general northeast and southwest direction along 

 almost innumerable lines and a black vein matter deposited. 

 The veins are generally small, varying from one- fourth to a 

 few inches in thickness, but in the case of the main tin-bearing 

 vein an enormous size is reached at Cajalco hill. As the hill 

 is approached the veins become larger, and finally culminate in 

 this elevation, which is about 300 by 250 feet in diameter at 

 the base. The veinstone of which it is mostly composed rises 

 in prominent and bold croppings. With one or two unim- 

 portant exceptions the material of which this as well as the 

 other veins is formed consists wholly of tourmaline and quartz, 

 with which the tin ores are locally associated. The larger 

 veins, and the Cajalco in particular, are very irregular in size, 

 sometimes appearing to be mere bunches in the granite. A 

 few hundred feet northeast of the hill the vein has narrowed 

 to 6 or 8 feet, and it is here that the large body of tin was first 

 discovered and the main shafts sunk. 



A slide prepared from one of the smaller veins, which in the 



