138 Dr Turner's Analysis of a Mica from Cornwall. 



the blow-pipe flame, I was induced to examine other varieties 

 of mica in the expectation of observing similar phenomena. 

 Through the kindness of Dr Brewster I was supplied with 

 specimens of twenty different varieties, among which were se- 

 veral from Scotland and Greenland, besides others from Coro- 

 mandel, Massachusets, Connecticut, Labrador, and Zinnwald. 

 All of these micas, with the exception of the last, were very 

 infusible, requiring the strongest heat of the blow-pipe to pro- 

 duce even imperfect fusion, the chief effect being loss of lustre. 

 But the variety from Zinnwald, on the contrary, fused readily, 

 frothed up during the continuance of the heat, and communi- 

 cated a fine rose tint to the flame. The only difference, in- 

 deed, between it and the Chursdorf mica is, that the latter gives 

 a white, the former a black bead by fusion. This observation 

 led me to examine the black mica of Altenberg, which accom- 

 panies Pycnite ; and it also was found to possess the characters 

 assigned by Professor Gmelin to the Lithion-micas. I now 

 mentioned these circumstances to my friend Mr Haidinger, 

 requesting him to examine the micas in the superb collection 

 of Mr Allan ; and he very soon succeeded in discovering seve- 

 ral which possess similar characters when heated before the 

 blow-pipe. Every specimen from Zinnwald, and several from 

 Cornwall, he found to be of this nature, and Dr Anderson of 

 Leith possesses a specimen from the Uralian Mountains, which 

 bears the closest resemblance to that from Chursdorf, both in 

 colour and fusibility. These facts render it probable, that 

 the Lithion-micas are by no means uncommon in nature, and, 

 to all appearance, several different species of them exist. The 

 comparison of their composition must, therefore, throw con- 

 siderable light upon the micas in general, and I have, on this 

 account, undertaken the analysis of a series of these minerals. 

 The subject of the present paper is a liver-brown coloured 

 mica from Cornwall. As far as regards the indications be- 

 fore the blow-pipe, it is the most unpromising mica I have ex- 

 amined ; but from not having a sufficient supply of other va- 

 rieties at the moment, I have been obliged to begin with it. 

 Though its texture is highly lamellar, like that of mica in ge- 

 neral, its laminae are small, possess little transparency, and are 

 inclined to break rather than bend when force is applied to 





