ADIRONDACK MAGNETIC IRON ORES 3 1 



long as the nature of the rocks themselves remained doubtful, the 

 problem might be viewed obviously from several standpoints. 



The sedimentary theory of origin has been held in most favor 

 perhaps by geologists. The condition precedent to its application 

 is that the inclosing formations are themselves of sedimentary 

 derivation. Different modifications of the general theory are possi- 

 ble: the deposits may be considered to have been laid down in the 

 form of magnetite, in which case they represent original surface 

 concentrations such as the magnetite sands that are found along 

 the shores of lakes and streams; they may have been originally 

 limonite or carbonate ores deposited from solution and subse- 

 quently changed under the influence of the metamorphism that 

 has affected the wall rocks. The apparent conformity between the 

 deposits and the foliation of the gneisses, their lineal development 

 and persistence for long distances on the strike are supporting 

 arguments for the sedimentary theory. 



In a previous paper on the Mineville deposits, 1 Professor Kemp 

 gave the first detailed account of the geological surroundings of the 

 magnetites. As a result of his investigations, he was led to ques- 

 tion the applicability of the sedimentary theory to the ores of that 

 district. The existence of igneous masses in the vicinity and the 

 evidences of their agency in the formation of many of the accom- 

 panying minerals were remarked and adduced in support of the 

 view that the ores have been introduced by processes connected 

 with the intrusion of those rocks, more particularly the gabbro of 

 Barton hill. In the present contribution it has been possible to 

 clear up some doubtful points relating to the geology of the dis- 

 trict, with the result that a more immediate source of the iron 

 minerals in the augite syenite is indicated. 



For the occurrences in the midst of intrusive rocks, which have 

 been found to be the prevailing type in the eastern Adirondacks, 

 there would seem to be no escape from the conclusion that the ores 

 have formed by igneous action. They are related to the wall 

 rocks just as the titaniferous ores are related to the gabbros and 

 anorthosites. 



The processes which led to the accumulation of these deposits 

 may have varied in some degree in the different localities. Mag- 

 matic differentiation has been, no doubt, a prominent factor in the 

 early stages of their formation and perhaps is competent to^explain 

 the whole course of their development. Yet there is reason for 

 believing that other agencies were active in producing, the final 



1 Geology of the Magnetites near Port Henry, N. Y. Am. Inst. Min. Eng. 

 Trans, v. 27. 1898. 



