346 GEOLOGY OF OLD HAMPSHIRE COUNTY, MASS. 



bands sometimes wautiug or exteudiiag only a little way iu, as if the rock 

 were but slightly affected by pressure. It is sometimes crowded with fine 

 black needles parallel to the axis c and with swarms of brightly polarizing 

 grains. 



The garnet is fresh, without inclusions or polarization, and j)artly idio- 

 morphic. 



The biotite is in aggregates of long, brown blades, with the usual very 

 strong absorption, sometimes accompanied by fine radiating wisps of mus- 

 covite. It is usually also associated with hypersthene, which is in stout 

 prisms, often showing the flat end faces. It is pale-green, without inclusions, 

 and shows, jc = ultramarine, b = reddish yellow, a = deep salmon color. 

 Some crystals are altered at one end into a green, negative, micaceous 

 mineral and change at the other into a brown-red serpentine. 



The dark-colored constituents form a very small portion of the rock. 

 The outlines of this occurrence can not be made out as it rises tlu'ough the 

 sands of a post-Glacial lake. It is one of a series of isolated stocks of 

 highly basic rocks, all very fresh and interesting for microscopical study, 

 which run north near the eastern border of the map, but mostly outside its 

 limits in Worcester County, including picrite, olivine-gabbro, and wehrlite. 



CORTr,A]VDITE. 



A single great mass of a brownish-black rock closely comparable to 

 one of the commonest tyi^es of the Cortland series occurs in the center of 

 the great tonalite area in the southwest corner of Belchertown, near D. 

 Griffin's. It is a hornblende-pyroxene-biotite-peridotite. The most strik- 

 ing pecviliarity of the rock is that at times it breaks up into angular blocks 

 with so great regularity that the fragments form rude rhombic dodecahedra 

 with faces about 2 inches across, and the surfaces of these blocks are cov- 

 ered with a thin layer of brown-red biotite scales, approximately j^arallel 

 and luster-mottled by grains of an emerald-green pyroxene. This pecu- 

 liarity is still more strikingly illustrated in the Cortland i-ock, and the 

 structure seems to replace a primary one, as in the deeper and fresher por- 

 tions of the rock the biotite is seen to be gradually encroaching on broad 

 surfaces of a dark hornblende which is finely luster-mottled with abundant 

 rounded grains of oli\ane and pyroxene. 



In the freshest slides the pale-brown, faintly pleochroic hornblende is 

 luster-mottled by rounded masses of olivine and more angular pyroxene of 



