156 MR. J. E. SCRITENOR ON THE [May I903, 



forms 120, 021, & 111. The vertical striation of the prisms and 

 the cleavage parallel to 001 are well developed. 



The mica is brown when viewed in its natural state, but when 

 powdered it exhibits a trace of the peculiar colour characteristic of 

 lepidolite. It never shows a distinct outline, and it was found 

 impossible to obtain a percussion-figure. The spectroscope gives a 

 strong reaction for lithium ; and the powdered mica is unaffected 

 by prolonged treatment with hot hydrochloric acid. The axial 

 angle is about 60° (2 E). Despite the colour of the crystals, 

 which is more characteristic of zinnwaldite than lepidolite, the 

 absence of a percussion-figure and the resistance to hydrochloric acid 

 make it unsafe to refer the mineral definitely to the former mica. 

 Indeed, since I have been informed by my friend, Mr. H. L. Bowman, 

 that it is sometimes impossible to separate zinnwaldite from 

 lepidolite, this mica must be described simply as a lithia-mica. 



The felspar at first suggested microcline or orthoclase ; but on 

 powdering a fragment very minutely and examining the structure 

 of the cleavage-fragments with crossed nicols under a high power, it 

 was found that it partook rather of the nature of a microcline- 

 perthite ; and that on similarly treating one of the pink felspars 

 from the granite (p. 148) the two were identical. 



Lithia-mica is a commonly occurring mineral in the quartz-veins 

 of greisens, and it is needless to quote authorities on that head : this 

 also is the case with topaz. The lithia-mica may have been formed 

 in situ by metasomatic action on the granite-face : the topaz may 

 have been formed similarly, or it may have been deposited by heated 

 water or vapour, percolating through, and deriving the necessaiy 

 material from, the freshly-formed greisen. This latter seems the 

 more probable view, since, from the poor development of the 

 greisen at the extremitj- of the tongue, it would appear that fluorine 

 was not present in sufficient quantities for the topaz to have been 

 formed by metasomatism on the wall of the fissure. 



As for the microcline-perthite, since we know that secondary 

 growths of felspar l occur both in igneous rocks and in sandstones, 

 as also we know that the recrystallization of felspar-grains has taken 

 place as a result of thermal metamorphism in arenaceous rocks, 2 

 it is not a matter for surprise to find that the pink felspar of the 

 granite has been redeposited, presumably by the same means as those 

 that effected the deposition of the topaz, in the ' bedding-planes.' 

 Nor, when we consider that the fluorine and boron were present in 

 less quantities at this particular spot, as is shown by the freshness of 

 the biotite and the poor development of greisen, is it a matter for 

 wonder that the felspar has been allowed to recrystallize here, but 

 not nearer the main mass of granite. The topaz and felspar were 

 probably deposited at the same time, approximately, as that when the 

 fissures of the * bedding-planes ' were rilled with quartz in the main 

 mass and the northern part of the tongue. 



1 C. E. Van Hise, Am. Journ. Sci. ser. 3, vol. xxvii (1884) p. 399. 



2 A. Harker, 'Petrology for Students ' 2nd ed. (1898) p. 286. 



