BIOTITE-GEANITE. 319 



gi'anite in the region. It is purely a l)iotite-granite, small-porphyrltic 

 in all its central portions. The feldspars are about three-fourths of an 

 inch long, rarely show carlsbad twinning, and are microcline without albite 

 bands. A few rounded spots, apparently of albite, break the continuity of 

 the cleavage surface. These felds})ar crystals are at times bounded by a 

 layer of secondary muscovite plates, and this is the only apjjcarauce of 

 rauscovite in the granite. 



The biotite is aggregated in groups of rather dull-l)lack plates, with 

 epidote, garnet, and rarely white apatite needles accompanying it. The 

 yellowish-white background is a somewhat friable mixture of much gran- 

 ular orthoclase and little bluish quartz, which is characterized by the 

 presence of small, elongate cavities. At the border the porpliyritic feld- 

 spars and the biotite aggregates disappear, and the friable ground with 

 small distant spots of biotite and the small cavities remain unchanged. 



THE COYS HILL PORPHYRITIC GRANITITE. 

 DESCRIPTION AND DISTRIBUTION. 



The Middlefield dike on the west of the area is matched by this still 

 larger dike on the extreme eastern border. It begins in Winchendon, and 

 runs south 25 miles across Phillipston, Barre, New Braintree, West Brook- 

 field, and Warren before it enters the Palmer quadrangle at its northwest 

 corner, and ends in Brimfield. Its whole length is 33 miles; its average 

 width is one-half mile. C)nly the portion in the Palmer quadrangle is 

 here studied. It is a highly feldspathic, very coarse-porphyiitic, garnet- 

 iferous granitite, which jiresents almost everywhere a distinct gneissoid 

 structure from the parallel arrangement of the large feldspars. It is jn-oved 

 to be an intruded rock by the fact that it runs for 20 miles in the Brim- 

 field fibroHte-gneiss; then, just as it enters the Palmer quadrangle, it crosses 

 very obliquely the Hardwick black granite, and enters the eastern band of 

 fibrolite-gneiss. This is further proved by the fact that where it sends 

 a great lobe into the western fibrolite-gneiss the boundary between the two 

 is a broad sigmoid curve, having a general east-west direction, while the 

 granitite on the north and the rusty fibrolite-gneiss on the south of this line 

 have the same foliation structure, which strikes N. 25° E. and dips 60°-70° 



