258 William s — Con tact-metam orphism near Peekskill. 



has a high angle of extinction. In different parts of the thin 

 sections, however, it is associated with different minerals. In 

 one place it occurs with a tri clinic feldspar, like that charac- 

 teristic of the adjoining diorite ; in another it is associated 

 with muscovite, and in a third with a colorless granular min- 

 eral, whose optical properties show it to be scapolite. Its 

 grains show sometimes parallel, sometimes rectangular cleavage 

 lines ; its refractive index is low, but its interference colors 

 (double refraction) very brilliant. Sections showing the rec- 

 tangular cleavage are isotropic or nearly so and exhibit in con- 

 vergent polarized light a negative uniaxial figure. The sub- 

 stance agrees perfectly in its appearance and optical behavior 

 with the scapolite described by Michel-Levy in the " gefleckter 

 Gabhro" from Bamle in Norway.* Sphene, often associated 

 with ilmenite, is also common in this rock. 



Section II (o-p on the map), shows essentially the same se- 

 ries of changes as those described under Section I. No. 25, 

 from the schist exposed at o, on the opposite side of the clay- 

 bank from the wall p, is the same mica schist as occurs at I 

 (Section I), with^ sillimanite and staurolite abundantly devel- 

 oped in it. (PL YI, iig. 3.) The quartz, biotite and muscovite 

 are like those described in No. 21. The tourmaline, however, 

 seems to be lacking in this section and the feldspar is rare. 

 The sillimanite occurs in the gray radiating tufts as described 

 inNo. 14; but the most abundant constituent of all is the 

 staurolite. This occurs in rather large and stout, more or less 

 perfectly rectangular sections, which possess a high index of 

 refraction. They show no well-developed cleavage, but have 

 an irregular cross-parting. The pleochroism is very marked, 



being parallel c, orange ; parallel a and b, pale yellow. The 

 staurolite individuals are often filled with inclusions of quartz 

 and magnetite. 



As we pass across the strike of the schists from o toward p 

 the contact (section II), the quartz steadily diminishes in quan- 

 tity, and biotite and sillimanite are proportionately increased. 

 Indeed, the mica schist finally becomes almost wholly a mixt- 

 ure of these two minerals, with a little garnet and magnetite. 



No. 36, collected a short distance north of No. 25, is largely 

 composed of brown biotite and gray tufted sillimanite, al- 

 though considerable quartz, muscovite and staurolite still re- 

 main. Garnets and occasional small zircons are also present 

 in this specimen. No. 37, however, from a point still nearer 

 the contact, has lost almost every trace of the quartz, musco- 

 vite and feldspar of the original schist. The staurolite too has 

 almost disappeared. Garnet, on the other hand, is largely de 



* Bull. Soc. Min. de France, i, 43, 1878. 



