PEGMATITE, SILEXITE, AND APLITE OF NEW YORK 41 



field evidence strongly points to a derivation of these minerals 

 from the older basic rocks by the pegmatites during their intrusion. 

 It is significant that more or less magnetite occurs in later pegma- 

 tites at every one of the dozen or more localities from which mag- 

 netic iron ore has been mined in the Lyon Mountain quadrangle 

 and also at many other places. A few of the more typical field 

 occurrences will now be briefly described. 



In the old mine at Russia Station pink granite and pegmatite 

 have been intimately injected into more or less hornblende gneiss 

 (presumably gabbro) , and much of the hornblende has been altered 

 or recrystallized into green monoclinic pyroxene, some of which 

 occurs locally in the form of coarse granular masses several inches 

 across. Magnetite is variably abundant in all the rocks, but it 

 is especially so in the coarser pegmatites and the silexite of later 

 origm which occur as rather well-defined dikes. 



On the ridge one and one-half miles northeast of Standish a very 

 irregular pegmatite dike, with a maximum width of thirty feet 

 and traceable for one hundred and fifty feet, lies in contact with 

 gneissoid gabbro. The pegmatite is moderately coarse to very 

 coarse and is very rich in magnetite, many masses of which are 

 from one to three inches across. Some tongues of the pegmatite 

 rich in magnetite cut across the foliation of the gabbro. 



On Elbow Hill two miles east-northeast of Lyon Mountain 

 village a pegmatite dike ten feet wide and thirty feet long is di- 

 rectly associated with a mixture of granite and gneissoid gabbro. 

 This pegmatite is practically a silexite, being very rich in quartz 

 and low in feldspar, and it contains much magnetite in masses up 

 to several inches across. The magnetite and quartz appear to be 

 of the same generation. Some of the more ordinary later peg- 

 matites close by contain Httle or no magnetite. 



In one of the ore pits near the western end of the main workings 

 at Lyon Mountain a pegmatite dike from one to two feet wide 

 and exposed for seventy-five feet rather sharply cuts intricately 

 mixed pink granite and hornblende gneiss. This pegmatite carries 

 little or no quartz, but it does contain some magnetite. 



By the river one mile south-southeast of Moffitsville later 

 pegmatite cuts intricately involved granite and hornblende gneiss, 



