84 The Sedimentary, Metamorphic, 



quartz-gangue was a stout prisin, about 5 x "25 inches 

 across the base, with the planes 



OP (001), oo P (110), oo Pn. (hko), ooPoo (100). 



These planes were imperfect and with rough surfaces, 

 excepting the basal pinnacoid, which, as usual, had a smooth 

 surface and a somewhat sub-metallic lustre. The prismatic 

 angles were near 120° and 60°. 



These particulars indicate that this pinite is a pseudomorph 

 after cordierite, but I cannot feel sure that all the other 

 examples which I extracted from this vein were alterations 

 of the same mineral species ; and still more must this be 

 doubtful as to pinite found in the rocks, for the numerous 

 thin slices which 1 have prepared of the Ensay Rocks * 

 show that other minerals have been pinitised, notably the 

 felspars, and most probably also magnesia-mica, very 

 extensively. 



I found this pinite, when examined under the microscope, 

 in a thin slice parallel to the basal cleavage, to have aggre- 

 gate polarisation almost uniformly throughout, but in places 

 there were small clear portions which obscured homo- 

 geneously. Numerous cracks traverse it, along which iron 

 ochre has been deposited. In places connected with these 

 cracks small divergent groups of colourless talc-plates have 

 been formed. When examined by ordinary light, and with 

 a high power, the slice is seen to be full of microliths, there 

 being stout, somewhat short fibres, some straight and some 

 curved. In places these are almost "felted." In other 

 places they are grouped together. 



A slice parallel to the prism was somewhat different to the 

 one just described. A large part of this is homogeneous, 

 and has straight obscuration parallel and perpendicular to 

 the basal cleavage. This mineral is colourless and very faintly 



i 

 dichroic, the ray vibrating parallel to the axis c being 



colourless, and the other (either a or b) being pale yellow. 

 The remainder of the slice shows aggregate polarisation, 

 and iron ochre stains parts adjoining cracks. With a high 

 power I observed the same short fibres arranged linearly 

 parallel with the basal cleavage in those parts which have 

 aggregate polarisation, but not in the homogeneous colourless 



* I prepared seventy thin slices of the Ensay Rocks for the purposes of 

 this paper. 



