158 ME. J. B. SCRIYENOR ON THE [May 1903, 



Lithia-mica has been formed in the fissures of the ' bedding-planes ' 

 in the tongue ; and topaz and microcline-perthite, which occur 

 in the mass, have been redeposited there by percolating water or 

 vapour. 



The greiseu is an example of Prof. Yogt's ' pneumatolytic ' action 

 on thoroughly acid rocks, 1 resulting in the formation of tinstone- 

 lodes, as contrasted with pneumatolytic action on syenitic rocks 2 

 with the production of zircon, etc., and on basic rocks, 3 as, for 

 example, olivine -gabbro, with the production of chlor-apatite and 

 scapolitization of the felspars. 



My heartiest thanks are due to those who have assisted me in 

 the course of the preparation of this paper, especially to Dr. J. S. 

 Flett, M.A., F.G.S., of H.M. Geological Survey. 



Discussion. 



Prof. Seeley said that the tourmaline-problem would require 

 examination over a wider area before the colour could be explained. 

 In the Portsoy district, where tourmaline-crystals were commonly 

 enclosed in quartz, the central part was sometimes brown and the 

 external part blue, or occasionally a blue crystal might have a brown 

 casing. Tourmaline was also abundant in pegmatite in the Pubislaw 

 Quarry at Aberdeen ; but here one side of a crystal might be brown 

 and the other blue, the limits of colour being irregularly defined. In 

 neither case was there evidence of such reactions as the Author found 

 at Cligga Head. 



Dr. Flett, while heartily congratulating the Author on his paper, 

 remarked that the questions with which he dealt were very interesting 

 and very difficult. In regard to the stanniferous veinstones, so far 

 as his experience went, the blue tourmalines were always later 

 than the brown. The problem of their origin was a large one, 

 and we required to know more before finally solving it. So far 

 as he could make out, the process of filling up tbe stanniferous 

 veins followed closely on the consolidation of the granite, and the 

 rock-succession was somewhat as follows : (1) pegmatite ; (2) greisen ; 

 and (3) second deposition of tinstone, quartz, etc. Copper-bearing 

 solutions arrived at a later stage of cooling, but all these processes 

 had long ago come to an end. 



The Author said, in reply, that he wished it to be understood 

 that he by no means regarded his inference concerning the tourmaline 

 in the lodes as final ; he himself had seen parallel growths of blue 

 and brown tourmaline, and did not venture to elucidate the occur- 

 rence, although he thought that an explanation founded on the 

 segregation of the magnesia, wherever it was derived, might prove 

 feasible. On this point, which was a secondary one in his paper, he 



1 Zeitschr. f. Prakt. Geol. vol. iii (1895) p. 145. 



2 Ibid. vol. ii (1894) p. 458. 



3 Ibid. vol. i (1893) pp. 4, 125, & 257. 



