322 GEOLOGY OF OLD HAMPSHIRE COUIfTY, MASS. 



are bounded by sides of the prism go P, which is tlie twinning plane. They 

 show a single axial ring, and the axial plane makes an angle of 48° with 

 that of (I), instead of 60° 50', as would be the case if they were cut par- 

 allel to P. The other crystal (III) is represented by a series of blades 

 which make an angle of 61° with the first series. They are also bounded 

 by "^ P, which is the twinning 2:)lane, and their optical axial plane makes 

 an angle of 58° on the other side of (I). The figure is drawn with crossed 

 nicols and with the crystal (II) at the point of extinction. 



MUSCOVITE-CiKANITE, OR PEG>LVTITE. 



One may omit any detailed description of a rock so well known. It 

 is a granular mixture of quartz, muscovite, and a potash-feldspar mostly 

 microcline, or orthoclase more or less mixed with microcline, and veined 

 with albite; generally coarse-grained, and often very coarse — a giant 

 granite with its feldspars several inches on a side and its mica a foot or 

 more across. Its greater masses and its smaller dikes are almost wholly 

 baiTen of any accessory minerals. In its dikes of medium or larger size 

 several minerals — as beryl, biotite, tourmaline, garnet — occur sporadically, 

 and much more rarely "secondary veins" of foliated albite contain these 

 and others of the rarest minerals in greatest beauty and profusion. 



The great masses which stretch from Montgomery to Conway and the 

 many dikes which go out from them, the great mass southeast of Mount 

 Toby in Leverett and the north of Amherst, and the dikes extending thence 

 south through Amherst and Belchertown are the main outcrops of this rock. 



These dikes have been extensively worked in Blandford and Hunting- 

 ton for mica, feldspar, and quartz, especially upon the propert^' of the 

 Pontoosic Flint Mills, in the north part of Blandford. There is opened 

 here one of the most beautiful veins of giant granite in the county. The 

 u[)per and the lower walls are occupied by a selvage 2 to 3 feet wide of 

 the coarsest muscovite, which projects inward with crystals a foot square, 

 mingled with feldspar. There follows above and below a layer 1 to 2 feet 

 wide of great feldspars a foot on a side, which projects freely inward. The 

 center, 2 to 5 feet wide, is of clear, smoky quartz, and in one direction the 

 whole vein changes to quartz. 



These veins are very rare in the tonalite, but going out from these 

 into the granites they increase gradually in number and at last swarm in 



