ON A BASIC ROCK DERIVED FROM GRANITE. 677 



explanation would account for the alteration of gneiss as well as 

 of granite, but does not remove the difficulty afforded by the 

 lack of metamorphism in the iron ore at the Sterling mine. There 

 are, moreover, other facts which need not be discussed here, indi- 

 cating that the ore is probably a secondary concentration younger 

 than the granite. Upon this supposition is based the second, 

 and, in the writer's opinion, more probable hypothesis to account 

 for the alteration of the granite. 



This hypothesis assumes that at the time of the granitic intru- 

 sion the ore had not been formed, its present place being occu- 

 pied by other rocks, chiefly limestone. The ore was formed by 

 the gradual replacement of the limestone, through the agency of 

 solutions which at the same time produced the alteration of the 

 granite. This explanation requires that a source shall be found for 

 the solutions supposed to bring about the whole series of changes. 

 It is believed that such a source exists in a ridge of gneiss which 

 rises a few rods west of the mine. The rock of this ridge is 

 highly pyritiferous and contains also much magnetite. As the 

 result of weathering, the surface becomes rusty, and the pyrite 

 almost wholly disappears, leaving the rock light and porous. 

 While this pyritiferous rock is not shown directly at the mine, 

 there can be no doubt of its presence, as its strike is such as to 

 carry it very close to the ore body. 



The oxidation of the pyrite yields solutions containing iron 

 sulphates and sulphuric acid. These solutions must be capable 

 of producing very marked chemical effects, and are just the sort 

 of agent required to account for all the phenomena under consid- 

 eration. Working down the dip and coming in contact with 

 limestone and granite, they would change the former to an iron 

 ore, with the consequent formation of solutions of lime and mag- 

 nesia sulphates. These solutions, as well as those derived directly 

 from the pyrite, would attack the granite, and, being very differ- 

 ent from the common agents of alteration, the product of their 

 action would naturally be of an unusual character, as is the case 

 with the rock under consideration. The solutions would have 

 much more powerful chemical action than the ordinary agents of 



