Williams — Contact-metamorphism near jPeekskill. 257 



up into radiating tufts at the extremities. They are arranged 

 in every direction and penetrate equally all of the other con- 

 stituents. Some of these tufts resemble in their appearance 

 the radiating tourmaline in the well-known Luxullianite. The 

 microscopic structure of the fibers is in all respects characteris- 

 tic* They vary from those of considerable thickness to micro- 

 scopic hairs. The garnet presents no unusual features. The 

 rutile forms often large but sharply defined crystals of a deep 

 red color. 



At a point ten yards from the contact {m on the map) on the 

 line of section I, the schist is very much contorted, although 

 still well bedded (No.. 14). The microscope shows that the 

 quartz and feldspar have been reduced to a minimum ; while 

 sillimanite, mica and garnet compose most of its substance. 

 The sillimanite is arranged in radiating tufts made up of need- 

 les so minute that their crowded aggregation is hardly trans- 

 parent even in the thinnest sections. Around the edge of these 

 masses, however, where the delicate fibers project into the sur- 

 rounding quartz or biotite, the characteristic structure of the 

 sillimanite is at once apparent. The grayish tufts are so abun- 

 dant as to compose a large proportion of the rock. Between 

 them is a rich brown biotite, frequent large flakes of musco- 

 vite, garnet grains and crystals, and some quartz. Feldspar is 

 rare, but opaque black grains of magnetite are thickly scattered 

 through the mica. This aggregation produces a striking con- 

 tact rock, but one which is quite constant along the whole ex- 

 tent of the Cruger's area. It is frequently modified by the 

 presence of staurolite, as we shall see beyond. 



Directly at the contact between the schists and massive rocks, 

 at a point in the wall marked m on the map (Section I), the 

 bedded rock appears to be more or less fused with the mica- 

 diorite, which here itself becomes very garnetiferous. t Even 

 where the bedding of the original schist is still distinct, a great 

 variety of mineral aggregates may be obtained. No. 18 b is es- 

 sentially a mixture of staurolite, garnet and biotite, containing 

 considerable quartz and feldspar. In this particular* specimen 

 there is very little sillimanite. No. 16, from this same expos- 

 ure, shows the actual contact with the diorite in the hand 

 specimen. The schist is here changed to an almost colorless 

 pyroxene, with some dark green hornblende and a little quartz. 



No. 18 d , a dark greenish rock from this same contact, proves, 

 upon microscopical examination, to be of unusual interest. It 

 contains a green diallage, in which both pinacoidal partings 

 are well developed. This is pleochroic as follows : c=b, bluish 

 green; a = yellow. This mineral occurs in rounded grains and 



* cf. Kalkowsky: Die Gneissformation des Eulengebirges, Leipzig, 1818, p. 5, 

 etseq., PI. I, figs. 1-4. 



