Williams — Contact-metamorphism near Peekskill. 259 



veloped, its crystals being penetrated by the biotite and silliman- 

 ite which compose most of the remainder of this rock. An- 

 other mineral, noted in this specimen for the first time, is 

 cyanite. This occurs in thick, colorless crystals, with the pris- 

 matic planes well developed, but without terminal faces. The 

 cleavage is marked by fine sharp lines parallel to the prism, 

 and the parting parallel OP, due to gliding, as shown by 

 Bauer,* is also very distinct. (See Pi. VI, fig. 4.) The ex- 

 tinction is in all sections greatly inclined. 



Specimen No. 27 shows the actual contact of the schist and 

 the diorite. Even the thin section is composed half of diorite 

 and half of the schist, which has here become, as in the case of 

 rocks similarly situated at m, an aggregate of quartz, feldspar, 

 colorless pyroxene and garnet (cf. Nos. 16 and 18 d ). Epidote, 

 zoisite and calcite also occur in this section. 



A study of the sections above described shows that there is 

 an undoubted metamorphism in the mica schist, which in- 

 creases regularly as the massive rocks are approached. This 

 metamorphism consists of an addition of alumina and iron and 

 the corresponding decrease in the proportions of silica and 

 the alkalies. This is shown by the disappearance of the quartz 

 and muscovite and the development of biotite, sillimanite, 

 staurolite, cyanite and garnet. 



The four following analyses, which were kindly made at my 

 request by Mr. Frank L. Nason of the Troy Polytechnic Insti- 

 tute, clearly indicate the nature of the chemical changes which 

 the schists have undergone. 





i. 



II. 



ill. 



IV. 



Si0 2 



62-980 



61-570 * 



55-120 



40160 



A1 2 3 



16-881 



19-529 



24-320 



29-500 



Fe 2 3 



2-479 



5-443 



6130 



19-660 



FeO 



5-000 



2-608 



4-990 



5800 



CaO 



trace 



trace 



trace 



trace 



MgO 



1-580 



1-900 



trace 



0-850 



Na 2 



3-020 



3-480 



2-712 



1-460 



K 2 



7-450 



2140 



2 826 



1-360 



TiO- 2 





1 530 



2-460 





S 



0-080 



0-846 



1-230 



0-820 



P 



trace 



trace 



trace 



trace 



Total, 



99-470 



99-046 



99-788 



99-610 



* Zeitschrift der deutschen geologischen Gesellschaft, xxx, p. 283, 1878. 



Am. Jour. Sci.— Third Series, Vol. XXXVI. No. 214.— Oct., 1888. 

 17 



