Vol. 59.] GRANITE AND GREISEN OF CLIGGA HEAD. 153 



that, if a rock be powdered up and gradually dropped into a 

 platinum-dish containing hydrofluoric acid, the glassy portions, if 

 any glass be present, will be first attacked, then the felspar, then the 

 quartz-grains, and lastly the ferromagnesian minerals. 1 From this 

 we may infer that the fluorine, in whatever form it was introduced, 

 would attack the orthoclase in the granite, and leave the biotite to 

 be decomposed by the less active, but no less important, factor in the 

 metasomatism of this granite, boracic acid. It is quite probable that 

 a little of the fluorine attacked the biotite simultaneously with the 

 boracic acid : Dr. Scharizers analyses show this to be so. But that 

 any of the boracic acid was able to attack the felspar in the face of 

 the fluorine seems just as improbable. 



It has been mentioned (p. 150) that the brown tourmaline is 

 often frayed out into blue acicular tourmaline : this may be in- 

 terpreted in one of two ways. Either minute traces of the original 

 felspar contiguous to the biotite have been spared from the fluorine 

 and incorporated in optical continuity with the brown tourmaline 

 by the boracic acid, certainly an improbable contingency ; or, and 

 this seems a more reasonable view to take, the decomposition of 

 the easily-cleavable biotite took place over the -whole surface of the 

 cleavage-plates simultaneously, and the magnesia and titanium- 

 dioxide segregated in the central portions of the tourmaline-crystals. 

 Whether those crystals of tourmaline which occur in other parts in 

 connection with the West-country granites (tourmaline which in cross- 

 section shows a band of blue and brown, or alternate bands of those 

 colours) admit of a similar explanation, is a point to be investigated 

 at some future date. 



Despite, then, the impossibility of determining by the secondary 

 silica-percentage in granite and greisen whether tourmaline has or 

 has not been formed from the felspar, in addition to muscovite and 

 topaz, yet in the light of the analyses given by Dr. Scharizer, we 

 may safely conclude from the foregoing considerations that the 

 tourmaline has all been formed from the biotite; and, 

 moreover, we may add as a corollary, that in a granite-modification 

 containing quartz, muscovite, and only brown tourmaline, we may 

 successfully look for topaz. 



It cannot be said, of course, that the brown tourmaline in the 

 killas has been derived from biotite, but there is a reasonable 

 suggestion in the connection that can be made. In the ordinary 

 course of metamorphism — that is, in the absence of fluorine and boron 

 at the margin of the granite — the product at the contact would have 

 been almost certainly brown mica. Now, knowing that brown 

 tourmaline can be derived from brown mica, we may explain the 

 presence of the former mineral in the killas by supposing ' potential 

 molecules ' of biotite to have been formed, which were prevented 

 from crystallizing by the interference of boracic acid ; or, to 

 put it more simply, the boracic acid acted on ' nascent ' biotite. 



1 Eo3enbusch [transl. Iddings] ' Microscop. Physiogr. of the Rock-forming 

 Minerals ' 1888, p. 108. 



Q. J. G. S. No. 234. m 



