Joly — On the Minerals of the Dublin and Wick low Granite. 55 



diams.) It polarizes in bright colours uniformly, and generally 

 appears limpid and clear, save for conspicuous cleavage streaks. 

 It is bordered where abutting on the felspar, with a dark margin, 

 due to difference of refractive index. 



It will be evident that if this constituent is beryl, and the 

 streaky lines alluded to basal cleavage, not only should we expect 

 simultaneous extinction, but we should expect it to occur when 

 these lines are in the plane of analyzer or polarizer, the axes of 

 elasticity of the section being then contained in these planes. On 

 trial it is found to happen so. 



Again, in the case of a section cut in a plane at right angles to 

 the one being described — that is, at right angles to the axis of the 

 prism — this same one of the two constituents should behave as if 

 amorphous ; that is, remain dark all round between crossed nicols. 

 I had a section cut in this direction from the same specimen, and 

 it behaved as expected, save that it showed the cross-hatched 

 appearance before alluded to as being noticeable on normal beryl 

 so cut. There was no appearance of cleavage. 



The analysis subsequently made confirming the presence of 

 beryl, it may be considered certain that this second constituent is 

 indeed that mineral. It contains no iolite. 



There is no crystallographic relation discernible in the distri- 

 bution of these two chief constituents, orthoclase and beryl. 

 Indeed, so far from such being apparent, the felspar seems to 

 wander at random through the beryl ; branching veins, sharply 

 defined and often of extreme fineness, spread over the field. 

 Rivers of felspar they look like — now widening into lakes 

 and again dwindling to mere streamlets. Scattered through- 

 out, the iolite glows with exquisite colour, like many- coloured 

 flower blossoms that have fallen and are borne along by a dark 

 river. 



Where broadest these veins are sometimes clouded over — a 

 muffed glass appearance — where probably the felspar is kaolinized 

 by water action. There is present also, chiefly through the beryl, 

 a chloritic mineral most nearly resembling Dana's prochlorite in 

 its habit — ropy, radiating, and vermiform. Some of these radiating 

 spheroliths — often extremely minute and closely crowded — show 

 the extinction cross with branches remaining along the sections of 

 the nicols as the stage is rotated, indicative of a structure radiating 



