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NEW YORK STATE MUSEUM 



whether varying specific gravity of the minerals is a factor in their 

 localization is yet to be demonstrated. The granulation of the mas- 

 sive rock and the gneissic banding are undoubtedly secondary 

 effects, having taken place after the consolidation of the rock as a 

 result of pressure. 



Garnet is the only undoubtedly secondary mineral present except 

 those which are subsequently caused by a local shear. The occur- 

 rence of such intense granulation without a corresponding change in 

 mineralogy (augite to hornblende or uralite, feldspar to saussurite 

 and albite, etc.) is unusual. Prof. Frank D. Adams, in the report 

 already cited, suggests that movement must have taken place while 

 the rock was deeply buried and at a high temperature. The deep 

 burying accounts for the absence of shearing effects ; the high tem- 

 perature for the lack of secondary hornblende, which needs low 

 temperature for its production. The Adirondack occurrence is pre- 

 cisely similar to the Canadian one here described. 



Gabbro. The gabbro proper is a basic variety, consisting of lab- 

 radorite, green monoclinic augite, titanite, sometimes hypersthene 

 and occasionally olivine. It usually presents an ophitic texture, with 

 broad laths of feldspar which have the ferro-magnesian constituents 

 between them. With increasing hypersthene the gabbros pass into 

 norites ; with increasing ilmenite and titaniferous magnetite the 

 gabbro passes into the titaniferous iron ores. 



The anorthosite and gabbro illustrate the familiar truth that basic 

 rocks are more liable to vary than acid ones. The gabbro family 

 appears to be particularly variable, as is evident from a comparison 

 of the mineralogy of the various types. The gradation from a pure 

 labradorite rock on the one hand to a titaniferous iron ore on the 

 other is a much greater change mineralogically and chemically than 

 is ever known in so small an area among granitic rocks. 



The gabbro area near Johnson pond presents a series of grada- 

 tions from a dark garnetiferous gabbro to a labradorite rich variety, 

 which is practically a pyroxenic anorthosite. In the northern part 

 of this area the more typical gabbro occurs, and its contact with the 

 anorthosite is distinct and suggestive of an intrusion of the gabbro 

 into the anorthosite. In the southern part, however, on Peaked hill, 



