T. Barron — A British Rock containing Nepheline, etc. 371 



The higher percentage of soda in the glass is probably due to 

 the artificial conditions ; there is a large overdose of soda, which 

 cannot crystallize. Michel Levy, in the analyses given above and in 

 others, finds that the silica percentage is greater in the spherulite, 

 which is exactly contrary to the result obtained by Lagario. In 

 this it will be seen that one of my analyses agrees with one 

 authority, and the second with the other. It is evident that the 

 differences between the matrix and spherulite may be small or large 

 and one way or the other, so that at present it is not possible to 

 prove any constant relation between them. 



The following papers will be found useful for reference, including 

 those given above : — 



Michel Levy. Bull. Soc. Geol. France, ser. 3, t. iii, p. 199 ; ser. 3, t. v, 



p. 232. Ann. des Mines, ser. 7, t. viii. 

 Delesse. Ann. des Mines, ser. 4, t. xvii, p. 116. Mem. Soc. geol., ser. 2, 



t. iv. 

 Bonney, T. G. Quart. Journ. Geol. Soc. 1885, Proc, pp. 88 and 90. 

 Potier. Bull. Soc. Geol. France, ser. 2, t. v. 



Wallerant. Bull. Soc. Geol. France, ser. 3, t. xvi, p. 927 ; (Note on large 



Vai-iolite), ser. 3, t. xvii, p. 447. 

 Rosenbuch. Zeitschrift der deutschen geologischen Gesellschaft, t. xxviii. 



Lagario. See Teall's "British Petrography," p. 397. 



IX. — On a new British Eock containing Nepheline and 



RlEBECKITE. 



By T. Barron, A.E.C.S. 



Introduction. 



THE mineral riebeckite was first discovered by Professor T. G. 

 Bonney 1 in 1882, who, in a paper to the Royal Society 

 describing a series of rock-specimens from the island of Socotra, 

 noticed the occurrence in a granite of a mineral which, though 

 presenting characters common to the hornblende group, he referred 

 doubtfully to tourmaline. 



Four years later Oebbeke 2 described a mineral from the island of 

 Sikoku, Japan, which showed intense colour and pleochroism, but 

 differed in other respects from glaucophane, to which he referred it. 

 The following year Professor A. Sauer, 3 of Leipzig, in examining 

 a series of specimens collected by Dr. E. Riebeck from the island of 

 Socotra, noticed the blue mineral described by Professor Bonney. 

 Having isolated and analyzed it, and determined its optical properties, 

 he placed it among the amphiboles, naming it riebeckite. In the 

 same year Rosenbusch 4 found a peculiar variety of hornblende in 

 a syenitic lamprophyre, which agreed in its optical properties with 

 the mineral described by Sauer. 



In 1888 Professor Bonney 5 described " a peculiar variety of 



1 Phil. Trans., vol. clxxiv (1883), p. 283. 



2 Zeitschr. fur Krysl, vol. xii (1886), p. 285. 



3 Zeitschr. der Deutsch. geol. Gesellsch., vol. xl (18S8), p. 138. 



4 " Die massige Gesteine," Bd. ii (1886-7), p. 312. 



5 Mineralogical Magazine, vol. viii (1888), pp. 103, 169. 



