A. R. Hunt — The Crystallimtion of Granite. 397 



tourmaline was derived directly from either of those minerals, and 

 came to the conclusion that Cornish tourmaline must be quite 

 different from Devonshire tourmaline. In trying to work out 

 Mr. Jukes-Browne's sands I found it absolutely necessary to come 

 to some conclusion as to the mica- derivation theory, and asked 

 Mr. Jukes-Browne to go through my slides with me, as my idea 

 was that the mica-derivation was not in evidence on Dartmoor, 

 whatever it might be elsewhere.^ To my great amusement and 

 delight I have since read Mr. Scrivener's paper, wherein he derives 

 the Cligga Head brown tourmalines entirely from mica ; so once 

 more the luckless crock gets among the pots. It is really a most 

 interesting question. As tourmaline is an aluminous mineral of 

 which boraoic acid is an essential constituent, there seems no reason 

 why, with the advent of boracic acid, tourmaline should not be 

 constructed out of any mineral that will supply the other con- 

 stituents, such as mica, felspar, slate, or clay. But as tourmaline 

 occurs on a large scale as massive schorl-rock, and as in Devon and 

 Cornwall mica occurs comparatively sparingly as scattered and 

 isolated crystals, and, further, as the crystals of tourmaline are often 

 large and those of mica always small, the amount and size of the 

 mica seem unequal to the task of supplying the bulk of the 

 tourmaline, at any rate as it occurs in Devonshire. 



For all practical purposes Devonshire tourmaline occurs as three 

 distinct minerals, viz., idiomorphic with the characteristic trans- 

 verse pseudo-cleavage, in fan-like bundles, and in needles and rods.. 

 The characteristic microscopic idiomorphic crystals are apt to show 

 a very marked longitudinal cleavage, which is entirely contrary 

 to ' Cocker.' A longitudinal splinter of tourmaline might be like 

 a flake of mica, but here the vertical versus the horizontal extinction 

 would distinguish the two. A transverse splinter of tourmaline 

 would appear to extinguish horizontally, but would differ from 

 mica in the pseudo-cleavage not being absolutely straight. The 

 passage of mica into tourmaline would be an interesting process to 

 trace, as the horizontal extinction would have to chang;e into 

 a vertical extinction. 



In his recent paper Mr. Scrivener has the following passages, viz. : — 



(1) " In discussing the origin of the brown tourmaline in 

 Luxullianite, Professor Bonney suggested that that mineral may 

 have been derived from the biotite of the granite ; . . . . 



(2) "The inferences which I have drawn from the study of 

 these slides is that all the brown tourmaline has been formed from 

 the biotite ; . . . . 



(3) " Professor Bonney suggested that the blue tourmaline in 

 Luxullianite has been derived from the orthoclase ;...." 



In my own slide, besides the innumerable greenish acicular 

 crystals, there is one compact crystal, of which one half is light 

 cobalt blue and the other half light brown. It is absolutely 

 characteristic in its transverse pseudo-cleavage, and I would 



1 Derivation from felspar I have since then occasionally noticed ; but my slides 

 do not seem to contain an indisputable case of derivation from mica. 



