176 W. H. HOBBS METAMOKl'IIISM OF BERKSHIRE SCHISTS. 



nar crystal of tourmaline has been formed and oriented like the core. 

 The dichroism of the enlargement is unlike the core, as it shows plum 

 tones approaching purple. Secondary enlargements of tourmaline have 

 been observed simultaneously by Whittle in the Green mountains and 

 are described elsewhere* in this volume. 



Porphyritic Biotite. — This mineral quite frequently appears in por- 

 phyritic blades, generally across the lamination. These are sometimes 

 several millimeters in diameter, and in the rock in which they occur are 

 several times larger than the sericite and biotite of the matrix. Small 

 sericite blades are generally inclosed in this porphyritic biotite. 



Ottrelite. — Ottrelite is found in the Mount Washington area, where its 

 minute disks occasionally spangle the surface of the schist. Under the 

 microscope the basal sections appear irregular and somewhat opaque. 

 Prismatic sections are lath-shaped and are sometimes broken across. They 

 show twins both simple and polysynthetic according to the Tschermak 

 law. The index of refraction is higher than that of chlorite, and extinc- 

 tion angles were measured as high as 17 degrees. The absorption is in 

 blue, green and yellow tones. The double refraction is feeble, causing 

 low gray interference colors. It is often difficult to distinguish this min- 

 eral from chlorite, which general^ accompanies it. Growths of ilmenite 

 similar to those described by Wolff in the New England schists f are not 

 uncommon, but they are here apparently of chlorite, possibly an alter- 

 ation product of ottrelite. Whittle X has shown that the ottrelite in 

 rocks from the Green mountains is altered extensively to chlorite. The 

 development of ottrelite in the Salmien superieur has been made the 

 subject of an extended memoir by Prof. J. Gosselet,|| of Lille. This 

 memoir is important because it throws a great deal of light on the mode 

 of development of other porphyritic constituents of metamorphic schists. 

 M. Gosselet concludes (p. 202) that the ottrelite was formed after the rock 

 was in the condition of a schist or phyllite, since all the constituents of 

 the schist are included in it. Further, the vicinity of the ottrelite crys- 

 tals shows an impoverishment of quartz. There has also been a local 

 movement of the particles around the ottrelite crystals at the time of 

 their development. The larger grains of hematite and ilmenite have 

 been forced out from the positions occupied by the ottrelite and concen- 

 trated in a zone around it. The diversion of the trains of inclusions in 

 the rock as they enter the ottrelite crystal, and the bending of mica scabs, 



*Ante, p. 152. 



t On some Occurrences of < >ttrelite and [lraenite Schist in New England : Bull. Mus. Comp. Zoo)., 

 vol. xvi, 1890, pp. 159-165. 



I An < ittrelite-bearing Phase of a Metamorphic Conglomerate in the Green Mountains : Am. Jour. 

 Sci., 3d sci-., vol. xliv, 1892, pp. 274-275. 



|1 Etude* sur l'origine de l'Ottrelite, ire Etude, I'Ottrelite dans 1'' Salmien superieur: Ann. Sor. 

 Geol. Nord, Lille, vol. xv, L888, pp. L85-318. 



