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XXXIV. 
ON THE OCCUEEENCE OE ZI^WALDITE IX THE GEAXITE 
OE THE MOUENE MOUXTAIXS. By Peofessoe W. J. 
SOLLAS, LL.D., E.E.S. 
[Re^ March 15, 1890.] 
The well-known presence of topaz among the beautifully crystallized 
minerals of the geodes of the Mourne Mountain granite led me to 
investigate the properties of the associated mica, which I conjectured 
might contain lithium. This mica occurs in well-formed crystals 
having the forms (001) (010) (111) for predominant faces, twinning by 
repeated superposition on the face c being common. The cleavage 
parallel to c is remarkably perfect even for a mica. 
A small cleavage flake when introduced (without previous prepar- 
ation) into the flame of a Eunsen's burner speedily fuses, and the flame 
is suffused with a superb crimson colour, evidently due to lithium. 
Examined with the spectroscope, in addition to the sodium and potassium 
spectra, the characteristic line (Li a) of lithium is seen, standing out 
with great intensity in the mid red. Thus there can be no doubt of the 
existence of lithium in the mica, and that in no inconsiderable quantity. 
In conjunction with my friend and pupil, Mr. Dixon, I am now en- 
gaged in making a complete analysis. When this is finished we shall 
hope to be able to state more definitely the proportion of lithium 
present. 
Different examples of the mica, and even different parts of the 
same crystal differ widely in colour. Dark green is common, and pale 
violet may occasionally be observed, but those examples in which the 
largest proportion of lithium is indicated are silvery white in 
appearance, and by transmitted light quite colourless. A beautiful 
zonal structure, parallel with the planes 010, 111, characterizes most 
of the crystals. In ordinary light this is apparent as thin bands of 
different colour running parallel to the edges of the six-sided cleavage 
plates. These become emphasized in polarized light between crossed 
nicols, except of course at the position of extinction, when the whole 
plate becomes uniformly dark. In crystals of more than one tint the 
