TONALITE. 337 



was originally erupted as a diallage-biotite-gabbro. It becomes apliauitic 

 in places, but is never porpliyritic or pegmatitic. It is a tough, compact 

 rock not easily disintegrated, but weathering white. It is light-gray, often 

 greenish, the two colored constituents rarely predominating, so as to give 

 it a dark-gTay shade, but often weathering so as to give it a greenish tint. 

 In other cases the feldspar weathers red, and it always has a somewhat 

 compact appearance, the cleavage hardly appearing. 



In the south of Belchertown a beautiful variety occurs abimdantly. 

 The quartz is amethystine, the diallage dark-bronze colored, the hornblende 

 bright-green. Very generally the decomposition of the hornblende has 

 furnished a large quantity of chlorite, which then gives a green shade to the 

 rock. 



Microscopical. — A description of the quartz-gabbro from South Bel- 

 chertown specimens will be given first, as the least altered form of the rock. 

 In thin sections the quartz shows fluid inclusions with moving bubbles. 

 Long, fine, rigidly straight, opaque needles of rutile occm- in great abun- 

 dance, and are often divided into many widely and regularly separated parts, 

 all perfectly aligned. The feldspar is nearly all triclinic, with extinction 

 of adjacent bands at 12° to 14°. Orthoclase could not be proved to be 

 present. The diallage is in separate, quite well-formed crystals of pale- 

 greeu color, but so loaded with the customary red and black inclusions as 

 to give it a deep-brown color. In sections jjarallel to oo P co (100) these 

 are, in abundance, shape, and an-angement, exceedingly like the Labrador 

 liypersthene, and the vertical striation is clearly developed. In sections 

 parallel to oo P cc (010) the red jjlates are not nearly so much shortened 

 parallel to the vertical axis as in the hvpersthene, and are so abundant 

 as nearly to obscure the green color of the diallage. 



In one regular octagonal basal section, while the diallage cleavage is 

 finely develoj^ed, and a cleavage less perfect and at right angles thereto 

 is clearly seen, the prismatic cleavage is entirely wanting. In another 

 twinned very clearly after the common pyroxene law, on go P go (100), 

 all the three cleavages are developed, the go P oo (100) cleavage being 

 nuich the best. The freshest of these crystals are surroimded by a nan-ow 

 zone of green, rounded plates of hornblende, in which the black inclusions 

 remain, but the red do not. 



MON XXIX 22 



