154 . ME. J. B. SCRIVENOK ON THE [May 1903, 



With regard to the occurrence of topaz in greisens, it was 

 found to be contained also in the greisen of the St. Agnes granite- 

 mass, and this greisen was found to be in other respects identical 

 with the Cligga rock, except for the fact that the original quartz- 

 grains have not yet been seen to contain the orientated prisms 

 of tourmaline. Macroscopic crystals of topaz were recorded by 

 Greg & Lettsom T in the killas of Wheal Kind (Wheal Friendly ?) 

 and Wheal Trevaunance ; and Mr. J. H. Collins 2 mentions them 

 also in slate at the Seal Hole tin-mine. All these localities are near 

 to the St. Agnes granite. Topaz similar to that of the Cligga and 

 St. Agnes greisens occurs in the St. Michael's Mount greisen, in 

 addition to the well-known macroscopic crystals in the * bedding- 

 planes.* Topaz in the Continental granite-modifications is well 

 known, but not always of similar origin. For instance, the only 

 topaz mentioned by Prof. W. Salomon & Dr. H. His 3 in the greisen 

 of Geyer occurs in nests, and is believed by those authors to be the 

 original topaz of the granite. 



The topaz-rock at Meldon. near Okehainpton, described by 

 Mr. Teall, 4 differs from the greisens in the abundance of plagio- 

 clase, in the absence of biotite, and in the green colour of the 

 tourmaline. 



It will be remembered that in a hand-specimen the original 

 quartz-grains showed a tendency to form a crystal-outline, and 

 that in section the blebs were seen to be generally surrounded by a 

 thin band of quartz, which resembled the secondary quartz derived 

 from the felspar. From the examination of the slides, there seems to 

 be no doubt that each individual grain has been built on to by some 

 of the secondary silica derived from the felspar and biotite, that this 

 quartz has been deposited in optical continuity with the original grains, 

 and that it has also produced the appearance of those grains being 

 bounded by poorly-developed crystal-faces. Secondary growths of 

 quartz in optical continuity in detrital deposits are well known. 

 Dr. H. C. Sorby noted them in 1880 5 ; the Rev. Dr. Irving 6 recorded 

 such growths in the sands above the Budleigh-Salterton Pebble-Bed in 

 1892 ; and my friend, Mr. H. H. Thomas, informs me that he has seen 

 similar detrital grains, which, by the addition of secondary quartz, 

 have formed dihexahedra. But secondary growths in the parent- 

 mass are not so well known. In the case of hornblende and augite, 

 instances have been recorded by Mr. Van Hise 7 ; and Prof. Judd 8 

 states that he found abundant evidence of the growth of quartz and 

 felspar at the expense of a more or less vitreous matrix — long after 



1 ' Manual of Mineralogy of Gi\ Brit. & Irel.' 1858, p. 221. 



a ' Mineralogy of Cornwall & Devon' 1871, p. 101. 



3 Zeitschr. d/Deutsch. Geol. Gesellsch. vol. xl (1888) p. 570. 



1 ' Brit. Petrogr.' 1888, p. 316. 



b Quart. Journ. Geol. Soc. vol. xxxvi (1880) Proc. p. 62. 



f; Ibid. vol. xlviii (1892) p. 71. 



7 Am. Journ. Sci. ser. 3, vol. xxxiii (1887) p. 385. 



8 Quart. Journ. Geol. Soc. vol. xlv (1889) p. 178. 



