26 NEW YORK STATE MUSEUM 



greatest variety of species. Professor Kemp 1 has listed the min- 

 erals from the former locality, with mention of their more 

 important characters. The Lyon Mountain locality has been 

 described recently in a detailed manner by H. P. Whitlock. 2 



As a rule the magnetites show little alteration or effects of weath- 

 ering, and are quite fresh at the surface. The only chemical change 

 at all common is oxidation with the formation of hematite. The 

 latter is usually pseudomorphic showing the characteristic granular 

 structure and octahedral parting of the magnetite — the form 

 known as martite. It occurs sparingly in several deposits, but in 

 quantity only on Arnold hill where the so called " blue " veins 

 are practically solid hematite. The oxidation of magnetite to 

 hematite is accomplished very slowly under ordinary atmospheric 

 conditions, and it seems to have been induced in these deposits by 

 some special agency connected probably with underground water 

 circulations. There are bodies of unaltered magnetite in the same 

 vicinity. 



Shape of the deposits 



The Adirondack deposits occur in a variety of forms such as 

 are common to the magnetites found in gneisses and schists else- 

 where. They have been designated by different writers as beds, 

 veins, pods; shoots, lenses etc., depending upon their particular 

 development in the locality investigated. 



In general the bodies have a much greater extent along the 

 strike and dip than at right angles thereto, and show a more or 

 less lenticular form in horizontal section, wider at the middle and 

 tapering toward either end. In some cases they are so prolonged 

 in the direction of strike that they are better described as tabular 

 bodies, their regularity of width being like that found in a bed or 

 stratum, a resemblance that has been emphasized by some geol- 

 ogists as evidence of a sedimentary derivation. The tabular and 

 elongated lenticular bodies are more abundant in the northern and 

 western Adirondacks. The Lyon Mountain, Arnold hill and St 

 Lawrence county districts afford examples. The greatest irregu- 

 larity of form prevails in the eastern districts, particularly those 

 of Essex county, where the deposits often exhibit a puzzling com- 

 plexity of pinches, swells and sharply compressed folds not observ- 

 able in other sections. 



1 Geology of the Magnetites near Port Henry. Amer. Inst. Min. Eng. 

 Trans, v. 27. 1897. 



2 Minerals from Lyon Mountain. N.Y. State Mus. Bui. 107. 1907. 



