r56 



NEW YORK STATE Ml'SEUM 



The inaiu difference in mineralogic make-up of the rock result- 

 ing from deformation is the production of abundant garnet. This 

 is usually a dark red garnet (probably a lime-alumina-iron garnet) 

 which occurs either as well formed crystals or as rims around 

 the magnetite (ilmenite). It occurs between labradorite and mag- 

 netite, less often between augite or hornblende and labradorite, 

 and never away from the feldspar. The material of which it is 

 composed has been derived from the adjacent minerals, particle 

 by particle, the garnet forming at their expense. As it is a more 

 basic mineral than the labradorite, an excess of silica is left, 

 which has crystallized out as quartz, partly inclosed in and partly 

 bordering the garnet. What has become of the soda of the feld- 

 spar is an interesting question. It would seem that albite should 

 Jiave formed, but it has not been detected. 



It is not easy to account for the fact that garnet has not been 

 produced between magnetite and feldspar in all cases, but there 

 are quite numerous instances of magnetite wholly surrounded by 

 feldspars, or partly inclosed by them, in which there is no trace 

 of garnet formation. Evidently some accessory to the process 

 was present in the one case and absent in the other. It does not 

 seem to have been water, since around some such magnetites the 

 feldspar is thoroughly granulated and has apparently been 

 largely recrystallized, implying the presence of water. The de- 

 velopment may have been confined to the cleavage planes, and 

 unquestionably is mainly in them. 



In those parts of the rock in which garnet is scarce or lacking 

 the magnetite is always completely inclosed by pyroxene and 

 hornblende. To a certain extent such hornblende seems to have 

 resulted from interaction of the magnetite and pyroxene. 



Between hornblende, or pyroxene, and feldspar garnet is by no 

 means so frequent as in the other association. It is in the more 

 gneissoid varieties of the rock that it is found so associated most 

 abundantly, here being well crystallized and strung out with the 

 dark silicates along the cleavage planes. Here unquestionably 

 much recrystallization has taken place, consequent on the deep- 

 seated deformation, and the garnet formation is to be ascribed 



