42 AXNALS NEW YORK ACADEMY OF SCIENCES 



products being antigorite and brown hornblende, or more rarely chlorite. 

 The light green antigorite forms in a confused fibrous mass over the 

 original mineral, the alteration starting from the border of the crystal. 

 The antigorite may form in considerable amount before the polarization 

 of the augite is destroyed. The brown hornblende forms in sharply de- 

 fined masses along the cleavage rocks, in strong contrast to the antigorite. 

 Calcite and quartz are accessory products. 



The orthorhombic pyroxene in these rocks is less ferriferous than in the 

 norites. Hypersthene in rather small rounded grains set in the augite, 

 and less altered than that mineral, is nearly always present. Its color and 

 pleochroism are usually not strong, however, and it grades into bronzite 

 and enstatite by a loss of iron and a concomitant decrease in refringence 

 and birefringence and in the optic angle about Z. These colorless varie- 

 ties are difficult of distinction from the white augite in plane polarized 

 light, but under crossed nicols, of course, the orthorhombic symmetry 

 becomes evident. Like the augite, the. enstatite is usually (but not al- 

 ways) allotriomorphic. Fine parting is generally well developed, and 

 the characteristic small inclusions, regularly arranged, are common. 



The rock is usually at least half made up of the augite, although the 

 proportions of the monoclinic and orthorhombic pyroxenes vary greatly. 

 Sometimes an almost pure enstatolite will be developed ; pure augite rock 

 is known, but hypersthenite is rare. The most common type consists of 

 augite to the extent of from one half to two thirds, with hypersthene and 

 enstatite — the latter usually the more abundant — for the other chief con- 

 stituent. Hornblende in small quantity, and generally basaltic, is very 

 common, especially in the Montrose Point area. Biotite is rarely present. 

 Traces of feldspar, chiefly unstriated, are not uncommon. Apatite is 

 practically absent. The chief ore is pyrrhotite ; pyrite and chalcopyrite 

 are less common; ilmenite and magnetite are always present, usually in 

 small quantity, and chromite is known. 



Green pyroxenite was found in only one patch, where it adjoins and 

 probably surrounds a limestone inclusion. It is located on the post-road 

 between Montrose and Buchanan. The available outcrops were all very 

 near the contact, the rock being greenish black and felsitic. Under the 

 microscope, it appears that the great bulk of the rock is composed of an 

 apple-green pyroxene in small crowded grains. The extinction angles run 

 from 30°-38°. In basal section a somewhat eccentric bisectrix figure is 

 obtained. The refringence is a trifle higher than usual in a pyroxenite, 

 and the birefringence lower. Pleochroism is more distinct in some grains 

 than in others, though it is nearly always visible : when most distinct X is 

 pale brownish, Y pale green and Z apple green, or X may be pale green. 



