678 THE JOURNAL OF GEOLOGY. 



weathering and would supply the elements which, as analysis 

 shows, have been added to the rocks. 



While it is impossible to trace this process with precision, 

 and there is no positive proof that it is what has actually occurred, 

 still the explanation has much to commend it. It accounts for 

 the association of the altered granite with the ore, and its 

 absence elsewhere in the region, by assigning the alteration of 

 the granite and the formation of the ore to a common cause. 

 It explains the very unusual character of the alteration as the 

 result of an unusual agent. Evidence of a general nature bear- 

 ing in the same direction is afforded by the fact, stated by 

 Roth, 1 that several of the hydrated silicates of iron and mag- 

 nesia, to which the green aggregate of the altered granite is 

 quite similar, are formed where the products of the weathering 

 of pyrite act upon silicates. These facts seem sufficient to war- 

 rant the tentative acceptance of the hypothesis as a reasonable 

 explanation of the phenomena observed. 



Similar rocks elsewhere i?i the region. — As previously stated, 

 the ore at all the mines of the region is associated with rocks 

 more or less similar to that at the Old Sterling, and generally 

 called serpentine. That these rocks have been subjected to a 

 process of alteration analogous to that of the granite has already 

 been suggested as probable, but it has not been found possible 

 always to determine their original character. At the Dixon 

 mine the "serpentine" is plainly an altered granite, like that of 

 the Old Sterling. The " serpentine " at the Clark and Pike 

 mines is an altered gneiss, but at the Caledonia mines its nature 

 is somewhat uncertain. Here it is an aphanitic mass, showing, 

 as a rule, no trace of its original minerals or structure. By Shep- 

 ard 2 it was included in his mineral species Dysyntribite, whose 

 variable nature was afterwards shown by Smith and Brush. 3 Its 

 relation to the ore is such as to suggest an intrusion, but the 



'J. Roth, Allgemeine und Chemische Geologie, Vol. I., p. 238. 

 2 R. U. Shepard, Am. Jour. Sci. (2) XII., p. 209. Treatise on Mineralogy, 

 p. 146. 



3 J. L. Smith and G. J. Brush, Am. Jour. Sci. (2) XVI., p. 50. 



