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



this mineral and the topaz show a slightly increased tendency to 

 do so. The tourmaline, however, even in thin slices, is frequently 

 elongated in the direction of the vertical axis (c') ; and as the 

 greatest absorption takes place at right angles, and compensation 

 with the quartz wedge occurs parallel to this direction (viz. to c'), 

 a ready and sure means of identifying the mineral exists in such 

 •cases. 



When this observation can be made one cannot remain in doubt 

 as to which is tourmaline and which is topaz, as the latter is not 

 dichroic in thin slices, and compensation with the quartz wedge, 

 in the case of topaz takes place at right angles to c', and in the case 

 of tourmaline parallel to c'. 



When dichroism is apparent in the Meldon tourmaline the change 

 is from colourless (e) to a yellowish or reddish-brown (o). 



In cases where the above-mentioned observation cannot be made, 

 namely, when dichroism is absent, I have found the following 

 methods very useful.^ 



I have often, when examining the topaz in my slides, been able 

 to obtain in converging polarized light the interference figure of an 

 optic axis, namely, a single bar, or a bar bisecting the first ring of 

 the interference figure, which remained without change of character 

 during a revolution of the crossed nicols through 360°. When this 

 interference figure is obtained it shows clearly that the mineral is 

 a biaxial one, and consequently that it is topaz and not tourmaline. 

 Axial sections of the latter mineral, on the other hand, may be 

 obtained in my thin slices, which in converging polarized light 

 yield a negative uniaxial cross which does not open out on revolving 

 the nicols. Sometimes when the section has not been cut quite 

 normal to the vertical axis of the crystal, only two of the arms 

 of the cross can be seen, and sometimes their point of junction 

 is outside the field. In such cases, with one of Swift's improved 

 one-sixth objectives, I can generally get the two arms just on or 

 just outside the edge of the field, and can make sure that the arms 

 are those of an uniaxial mineral. In cases of doubt I am able to 

 confirm this observation by inserting the one-fourth undulation mica 

 plate, when the double i-efraction of tourmaline being negative, 

 one dot (only half of the cross being visible) appears in a line with 

 the axis of the mica plate and well within view. 



Observation of the strength of the double refraction also afibrds, 

 in some cases, a method of distinguishing between the two minerals. 

 The birefringence of topaz does not exceed 0010, whilst that of 

 tourmaline is 0-020. The colour of the former mineral, as seen 

 in thin slices, does not rise above the indigo-blue of the second 

 order of Newton's scale, and very often falls below that. On the 

 other hand, the colour shown by the tourmaline in my sections 

 frequently rises to the indigo-blue of the third order. 



Lastly, the character of the metamorphism efi'ected by aqueous 

 agents afibrds a useful test. Tourmaline alters to steatite, mica, 



1 I confine myself to optical tests and those which can be applied to thin slices. 



