320 GEOLOGY OF OLD HAMPSHIRE COUNTY, MASS. 



west with great uniformity, and thus cuts directly across the boundary 

 between the two rocks at every angle. The sudden disappearance of 

 gi-aphite, pyrite, and fibrolite, and rustiness and the great increase of feld- 

 spar, separate it from the Brimtield gneiss. 



This gneissoid structure, furthermore, distinguishes this dike from the 

 other granites of the region and indicates for it a greater age. The other 

 granites have often been injected into the vertical foliation planes of the 

 schists after these had been completely formed, and do not show any 

 trace of having been subjected to the pressures which have given these 

 sti-uctures to the schists, while here the granitite and the schists have been 

 subjected to the same compression. 



A crushing of the feldspars occurs in the bordering portions of the 

 granitite itself, and is well shown where, across the brook north of Fenton- 

 ville, in Brimfield, the western boundary runs up the mountain side. Here 

 the large feldsjiars are only slightly rounded and the sugary border of 

 crushed feldspar still retains the angular boundary of the former crystal. 

 The change increases until only the transparent centers remain, and this 

 causes a marked whitening of the whole rock and ends with the formation 

 of a light-gray, granular graiiitite, hardly to be distinguished from the 

 Monson gneiss. This forms a selvage to the dike a hundred feet wide near 

 H. Sherman's, a mile southwest of West Wairen, and a large quarry has 

 been opened upon the same rock on the west slope of Colonels Mountain, 

 in the northeast corner of Palmer. 



The rock can well be described by supposing the lai-ge porphyritic 

 carlsbad twins which are scattered through the rusty fibrolite-schist of East 

 Monson and Sturbridge to develop so abundantly that a complete augen- 

 gneiss should result, the biotite and the garnet remaining the same as in 

 the fibrolite-schist, and only the fibrolite, graphite, and pyrite disappearing, 

 which they do almost uniformly. 1 have been thus led at times to consider 

 this rock an extreme of the granitic impregnation which has affected the 

 fibrolite-schists in this region, and not an intruded plutonic rock pure 

 and simple. A gi-anite dike 33 miles long and only 2,500 feet wide is 

 rather anomalous, especially in a region where the granites are in great 

 blocks of a wholly diiferent type. We are here, however, at a jioint 

 where the type changes. Farther east porph^rritic granitites are very 

 common. 



