376 mk. J. e. scriyexoh ox the [Dec. 1914* 



were certainly formed by the alteration of pre-existing rocks, 

 markedly different from the quartz-topaz rock (see the points of 

 difference enumerated above) ? 



Fig. 3 (p. 366) shows better than any verbal description how 

 the quartz-topaz veins occur, and that sketch, with the evidence 

 just adduced, must, I think, sweep away all doubt about the rock 

 having consolidated as it is now with quartz and topaz as original 

 constituents. 



The origin of the rock led to a difficulty in assigning to it a 

 name. I have referred to it in local publications as the ' Chin- 

 chong rock' or the ' quartz-topaz-cassiterite rock' ; but, after seeing 

 it in situ, I sought for a better appellation. Where the iron-rich 

 zinnwaldite occurs the rock might be called a greisen, indeed a 

 very similar rock has been figured as greisen. 1 But, although 

 greisen is not necessarily an alteration-product, 2 most of the well- 

 known examples have been formed by pneumatolytic changes, and 

 the chief minerals of greisen are quartz and mica, the mica being 

 usually white. This is not a fatal objection to calling the rock in 

 Quarry M greisen, but that name cannot be applied to the other 

 veins, where mica as an original constituent of the rock plays 

 a part less important- than apatite in granite. By no stretch of 

 imagination can the main constituents of the A T eins, apart from that 

 in Quarry M, be said to be quartz and mica, and therefore the 



1 R. Beck, ' The Nature of Ore-Deposits' [Engl, transl.], vol. i, p. 200. 



2 See A. Harker, ' Carrock Fell : A Study of Variation of Igneous Rock- 

 Masses— Pt. II, The Grainsgill Greisen,' Q. J. G. S. vol. li (1895) pp. 139- 

 47. On p. 142 the author states that ' these Cumbrian quartz-mica rocks 

 differ in some respects from typical greisens, such as those of Cornwall and 

 Saxony.' On pp. 142, 143 the author writes of the ' topaz, tinstone, and other 

 minerals characteristic of the true greisens.' That these minerals occur in 

 connexion with greisens is of course admitted, but I think that their im- 

 portance is now somewhat overestimated. Dr. J. S. Flett, in the ' Geology of 

 the Land's End District ' (Mem. Geol. Surv. Expl. Sheets 351 & 358, 1907) 

 gives an account of some Cornish greisens (see pp. 57, 58, 66). He says on 

 p. 57, ' Topaz is very commonly, though not constantly, present.' On p. 66, 

 in the description of the ' greisening ' of el vans there is no mention of topaz, 

 bat tin-ore is said to occur. Dr. W. Lindgren in ' Metasomatic Processes in 

 Eissure-Veins ' (Genesis of Ore-Deposits) says, on p. 540, that greisen is a 

 granular rock consisting- chiefly of quartz, topaz, and white mica, thus making 

 the topaz more important than the mica ; and, on p. 544, the same author 

 says that the name greisen ought to be reserved for the granular alteration- 

 products of granite consisting of ' quartz, lithion-mica, topaz, and cassiterite.' 

 It is difficult to agree with this. Reference to text-books will show that 

 greisen is generally regarded as consisting essentially of quartz and mica. 

 The other minerals play the part of accessories. F. Zirkel ('Petrographie ' 

 2nd ed. 1894, vol. ii, p. 122), however, regards the lithia-mica also as an 

 accessory, and the same view is taken by H. Rosenbusch (' Mikroskopische 

 Physiographie der Massigen Gesteine ' 3rd ed. 1896, p. 83). These authors 

 regard the change from felspar to quartz as the most important feature of 

 the rock, but the Cornish examples show that the production of secondary 

 mica is equally important. I have a specimen in my office of Zinnwald 



-en, obtained from a dealer. It is not like the Cornish greisens, but 

 sembles the rock in Quarry M on Gunong Bakau. The amount of topaz 

 is small, and what is present is closely associated with the zinnwaldite, as in 

 the Gunong Bakau roek. 



