MUSEUM OF COMPARATIVE ZOOLOGY. 161 



find any outcrop of it." ^ The carboniferous schists abound in little 

 black metallic plates which resemble ottrelite, so that the rock may 

 have been mistaken for ottrelite schist ; but since pebbles of this rock 

 ai'e associated with pebbles of the true ottrelite schist, there is little 

 reason to doubt that the latter occurs in place in the vicinity, and prob- 

 ably of Carboniferous age.^ 



Ottrelite Schist. — The ottrelite schist here described was collected by 

 Mr. Dale, occurring as pebbles on Easton's Beach, Newport. The rock 

 is a silvery-gi'ay, fine-grained mica schist, which has a well marked 

 schistosity (and cleavage), the plane of which bears no relation to the 

 distribution of the ottrelite. This mineral occurs in the well known 

 rliomboid or irregular plates, three or four millimeters in diameter, 

 with brilliant lustre and well marked cleavage surfaces. The latter are 

 pitted with little dull spots, which it is seen in the slide are grains of 

 quartz enclosed by the crystal. 



Studied in the thin section, the rock is found to be composed of little 

 rounded grains of quartz, closely interlocking, when not separated by 

 the otlier constituents, and of minute scales of colorless mica with tlie 

 optical pi'operties of muscovite, which by their parallel ai-rangement 

 cause the schistosity of the rock. Certain wavy lines oblique to this 

 structure, which contain less mica and more quartz than the average, 

 may represent the original plane of deposition. A darker variety of the 

 rock contains occasional small plates of chlorite and bands of opaque 

 black substances, which are mixtures of graphite and titaniferous iron 

 ore (ilmenite'?) for the powdered rock gives a strong test for titanium 

 and also for graphite. 



The ottrelite crystals and somewhat smaller black metallic plates are 

 seen to have no connection with either the plane of schistosity or 

 possible deposition plane. The former mineral occurs in plates of 

 irregular outline, appearing as lathe-shaped cross-sections, frequently 

 twinned several times, with composition parallel to the base, blue and 

 greenish plcochroism, and the other usual optical properties. They are 

 generally tilled with little grains of quartz of the same size and shape as 

 those composing the rock outside, which were evidently enclosed by the 

 crystal as it formed ; it is noticeable that the muscovite never accom- 



1 A contribution to tlie Geology of Rhoile Island, Am. Journ. Sci., Vol. XXVII. 

 p. 222. 



- Mr. Dale has found ottrelite seliist in place on Conanicut Island, opposite New- 

 port, but the rock has not been examined microscopically. Proceedings of Cana- 

 dian Institute, 1884-85, p. 21. 



