676 THE JOURNAL OF GEOLOGY. 



by the removal of certain constituents of the granite, and the 

 addition of a great amount of iron, with less magnesia and much 

 water. The association of the altered granite with iron ore, sug- 

 gests that the latter is in some way connected with the process 

 of alteration. That such a connection does exist is shown by 

 the fact that rocks similar to the altered granite, and sometimes 

 of like origin, occur at all of the ore mines of this vicinity ; 

 while, on the other hand, nothing of the kind is found away 

 from the ore, although granite is a very common rock. 



Accepting the connection between the alteration of the 

 granite and the presence of the iron ores, two hypotheses are 

 suggested to account for the phenomena. The discussion of 

 these two hypotheses involves the whole question of the origin 

 of the iron ores, which is considered at some length in the 

 report to which reference has been made. For present pur 

 poses only a very brief summary of the most salient points is 

 necessary. 



According to one hypothesis the granite is younger than, and 

 has been intruded into, the iron ore. As a result of this intru- 

 sion the granite has undergone marked endomorphic changes, 

 during, and subsequent to, the intrusion, becoming heavily 

 charged with iron and assuming its present form. But this hypoth- 

 esis is rendered doubtful (aside from its inherent weakness) 

 by the absence of any contact phenomena in the iron ore, and 

 by the presence of an analogous serpentine-like rock at another 

 ore mine, which is derived from a finely laminated gneiss, instead 

 of a granite. In fact, there is nothing to indicate that the change 

 in the granite associated with iron ore is, in any way, the result 

 of the action of heated solutions generated at the time of 

 intrusion, while much evidence is at hand to prove that this is 

 not the case. There is more probability in a modification of this 

 hypothesis, by which it is assumed that the ore was originally 

 siderite and that oxidizing meteoric waters changed it to the 

 peroxide, with the production of much carbon dioxide, which, 

 being carried in the percolating waters, might be a sufficiently 

 powerful agent to bring about the alteration of the granite. This 



