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NEW YORK STATE MUSEUM 



ahedron modifying the edges. Locally and at the time of pro- 

 duction for many miles up and down the Delaware and Hudson 

 Railway the crystals were known as " diamonds.'^ During visits 

 to the mine the wrHer made a careful study of hundreds and 

 endeavored to detect other faces, freely using the reflecting gonio- 

 meter in the measurements ; but all the determinations led to such 

 extraordinary indexes and to such variable results that the faces 

 were believed to be merely interference planes produced by con- 

 tact. The plane faces were found to be traversed by regular 

 series of striations most of which follow the octahedral parting 

 planes, but others are parallel to still other faces as described in 

 the reference below to the writer's paper on '' Gestreifte Magnetit- 

 krystalle/' 



The finest of all the crystals of magnetite from Mineville is 

 preserved in the office of Witherbee, Sherman & Co. at the mine 

 and is about an inch in diameter. It is almost a mathematically 

 perfect octahedron, having only one slight interference plane on 

 one apex. Fortunately the matrix is also preserved but the crystal 

 is believed to have come from the Old Bed (or Sanford) pit. 



All the pits are from time to time sources of cleavage pieces 

 boun-ded by octahedral planes and often of large and regular size. 

 The apparent cleavage is, however, really due to a series of parting 



Fig. 3 4 Magnetite crystal from the Split P.ock mine 



planes or gliding planes as is usually believed to be the case with 

 minerals of the spinel group. Rarely these plates exhibit brilliant 

 luster. 



In the coarse pegmatite of the "21 " pit, moderately large but 

 -\ery fragile crystals of magnetite are not uncommon, which are 

 dodecahedral forms built up of octahedral triangular planes, a very 

 common feature of magnetite. 



