112 J. F. Kemp — Great Shear-zone in the Adlrondacks. 



sive hornblende schist or of a gneiss. Chips were taken from 

 the edge of the zone and at intervals of ten feet across. They 

 show some slight variations in mineralogical composition but 

 they are essentially all the same rock. There is no evidence of 

 the common chilling along the walls and coarser crystallization 

 in the center, such as we see in dikes, but the structure is quite 

 uniform throughout. The rock consists of broken irregular 

 masses of plagioclase, O'l-O'S 111 " 1 , of shreds of hypersthene, green, 

 non-pleochroic, monoclinic pyroxene, pink garnet, greenish- 

 brown hornblende, biotite and magnetite. The individual 

 minerals are nearly in this order of abundance, but in the 

 aggregate the dark silicates far surpass the feldspar. This is 

 shown in fig. 3. The first four named, including plagioclase, 

 are in excess. The plagioclase frequently shows abundant evi- 

 dence of crushing and indeed as fig. 3, indicates all the 

 minerals are irregular and broken. Fig. 3, was drawn with a 

 camera lucida. The actual field is 4 mm . The impossibility of 

 indicating in faithful line work the differences of the minerals 

 has prompted putting initials on them. G is garnet; F, feld- 

 spar ; A, amphibole ; H, hypersthene ; B, biotite and the 

 hachures are uniform for each. 



The hypersthene with pink to green pleochroism repeatedly 

 passes into garnet which has a tint much like its pink. The 

 same cracks run through both and it is not always possible to 

 be sure when the pink shows, which mineral is present, until 

 either the pleochroism or the isotropic character is proved. 

 Two such are shown in fig. 3, one in the center, one in the 

 lower right-hand quadrant. Pleochroic hypersthene passes in 

 a similar way into non-pleochroic pyroxene. The same cleav- 

 age cracks pierce both, but the extinction angles are different, 

 rising in the latter to various angles from 27°-44°. In in- 

 stances the hornblende seems also to have resulted from the 

 hypersthene. Biotite favors the neighborhood of magnetite, 

 but in fig. 3 is shown cutting a garnet mass in two. It is com- 

 paratively rare. 



It may be said, therefore, that in the walls we have a coarsely 

 crystalline rock, containing plagioclase, hornblende and mag- 

 netite, but chiefly the first named. In the shear-zone, the dark 

 silicates, hypersthene, green monoclinic pyroxene and garnet 

 are collectively most abundant and with them is considerable 

 plagioclase, a little hornblende, biotite and magnetite. The 

 hypersthene passes into the monoclinic pyroxene and into 

 garnet both of which are considered to be derived from it. 

 That the feldspar has also helped to furnish these minerals, 

 probably by contributions of calcium and aluminium, is a well- 

 nigh irresistible conclusion from the way it is distributed in 

 isolated patches through them, Dynamic effects are abundant. 



