152 MR. J. B. SCMVENOB ON THE [^&y I903, 



from the analyses quoted by Prof. S. L. Penfield and Mr. H. W. Eoote, 1 

 brown tourmalines are shown to be essentially ' magnesia-tourma- 

 lines ' ; dark brown, ' magnesia-iron tourmalines ' ; and it will be 

 seen from the analyses of Dr. R. Scharizer 2 that the percentage of 

 alumina is slightly greater in blue than in brown tourmaline ; the 

 percentage of magnesia and lime much more in brown than in blue ; 

 the percentage of alkalies slightly greater in blue ; and that there 

 is no fluorine in the blue specimens quoted, while a very small but 

 calculable quantity is present in some brown tourmalines ; also 

 that titanium is only quoted in one blue, while it is constant and 

 reaches as much as 0*37 per cent, in the brown specimens. 



The presence of the minute, pale-blue prisms in the original 

 quartz of the Cligga granite proves that tourmaline was present in 

 the magma as an original constituent : there is nothing very extra- 

 ordinary in this, for Dr. Karl Dalmer 3 has shown that in the 

 granites of .Schellerhau and Altenberg topaz is found as a primary 

 constituent ; Dr. W. Salomon & Dr. H. His 4 have noted the same 

 fact in the case of the greisen of Geyer ; and Mr. J. J. H. Teall 5 writes 

 that it must be admitted that in many cases the tourmaline plays 

 the role of a normal constituent. Again, there is no evidence to 

 show that the blue tourmaline in the lodes has been derived from 

 any other mineral, and Prof. Bonney further noted the fact that in 

 a schorl-rock at Mousehole ° the blue tourmaline-prisms surround- 

 ing the more massive brown tourmaline-crystals are formed when 

 felspar-crystals are contiguous to the spot occupied by the brown 

 tourmaline. It may, therefore, be taken as at least very probable 

 that all wholly blue tourmaline in granite-modifications and lodes 

 has either been derived from orthoclase, or crystallized in its present 

 state directly from the magma, whether fluid or vaporous. 



In the Cligga granite there have been two main processes of 

 decomposition — orthoclase affording muscovite, topaz, and quartz ; 

 biotite affording brown tourmaline, magnetite, and quartz : can any 

 of the tourmaline have been derived from the felspar ? In the case 

 of orthoclase 49 per cent, of silica has separated out as quartz; in the 

 case of biotite, as may be seen from a comparison of the silica- and 

 alumina -percentages in biotite and tourmaline, roughly 17 per cent, 

 only could have separated, If any tourmaline had been formed 

 from the felspar, the amount of secondary silica in the greisen 

 would be approximately the same ; so that no comparison between 

 the silica-percentage in the granite, even were it fresh enough, and 

 that in the greisen would help in deciding the question. We know, 

 however, that the most active reagent introduced into the granite 

 has been fluorine, and that it was present in considerable quantities 

 is evident from the abundance of topaz. Now, it has been proved 



1 Am. Journ. Sci. ser. 4, vol vii (1899) p. 97. 



2 Zeitschr. f. Krystallogr. vol. xv (1889) p. 337. 



3 Zeitschr. f. Prakt. Geol. vol. ii (1894) p. 320. 



4 Zeitschr. d. Deutsch. Geol. Gesellsch. vol. xl (1888; p. 570. 



5 ' Brit. Petrogr.' 1888, p. 315. 



c Min. Mag. vol. i (1877) p. 219. 



