Lieut.-Gen. McMahon — Tourmaline of White Granite. 317 



White tourmaline (achroite) has not yet, so far as I am aware, 

 been found in the British Islands, but the tourmaline of the Meldon 

 granite approximates to colourless tourmaline sufficiently closely to 

 render it probable that if the attention of mineralogists is drawn to 

 the subject true achroite may yet be detected in British rocks. 



The following remarks are based on the study of a good suite 

 of thin slices made from hand specimens collected by me, and of 

 numerous fragments of tourmaline separated from these specimens 

 with the aid of a heavy solution. When I was at Meldon the 

 granite was being quarried, and I was able to get unweathered 

 samples. 



Fragments of tourmaline examined with a powerful pocket lens 

 are seen to be in part colourless and in part of pale-brown colour. 



Under the microscope, wlien examined with the aid of transmitted 

 light, the tourmaline in thin slices (as thin as those " made in 

 Germany " for instance) is absolutely colourless and devoid of 

 dichroism. It is consequently difficult to distinguish from the topaz 

 with which it is associated. This difficulty is increased by the fact 

 that both minerals, as seen in thin slices of the Meldon rock, closely 

 resemble each other in habit. Both are allotriomorphic and occur 

 in irregular shaped grains. The tourmaline rarely presents itself in 

 prismatic form, whilst the cleavage, usually so characteristic of topaz, 

 is rarely to be seen in the topaz of the Meldon granite. As, moreover, 

 the refraction of both minerals is higher than that of Canada balsam, 

 and of the felspar and quartz with which they come in contacV 

 the difference in the refraction of topaz, as compared with that of 

 tourmaline, does not help one to discriminate between them. 



The positive character of the double refraction of topaz could not 

 be made out in any of my slices, as no bisectrix could be seen in 

 converging polarized light. The axial angle in topaz from different 

 localities varies very much, and it is probably large in the variety 

 found in the Meldon granite. 



The rock under consideration affords an illustration of the help 

 that may sometimes be afforded by our much abused thick slices 

 of English manufacture. In such slices the Meldon tourmaline 

 presents a somewhat more normal appearance. Even in thickish 

 slices, however, the mineral is sometimes colourless in whole or in 

 part, but more often a reddish or reddish-brown streak or patch is 

 to be seen in the otherwise colourless grains which exhibits the 

 characteristic dichroism of tourmaline. 



The appearance of these coloured stripes and patches suggests 

 to me the possibility that the colour may be due to the alteration of 

 an originally colourless tourmaline ; namely, to the oxidation of the 

 iron contained in the mineral. Iron is not an essential constituent 

 in this complex silicate, and my previous studies have familiarized 

 me with the idea that iron may bo removed, or oxidized, without 

 breaking up the fundamental silicate of which it is more or less 

 a casual unessential member. 



The tourmaline in the Meldon granite very rarely exhibits 

 prismatic outlines in thin slices, but iu the isolated fragments both 



