MICAS IN GNEISS AND LIMESTONE. 



365 



which Heddle states to be formed of oligoclase, quartz, and Haugh- 

 tonite, with some orthoclase. It is considered likely that lepidome- 

 lane is the dark mica of some gneisses, such as the bronzy gneiss of 

 Tiree, while Haughtonite occurs in others. Haughtonite is readily 

 distinguished from biotite under the blow-pipe, since biotite whitens, 

 and Haughtonite increases in blackness. 



WHITE MICAS. 



BLACK MICAS. 



