62 Scientific Proceedings, Royal Dublin Society. 



of tlie prism. It appears to me most probable that they are 

 some form of andalusite ; in addition, very numerous grains and 

 plates of quite irregular form are present, and occasionally cavi- 

 ties, which are empty or filled with air : large opaque black 

 grains, probably ilmenite, are not infrequent inclusions. 



Green Mica. — The wide range in specific gravity of this con- 

 stituent (from 3"01 to 2*795) is no doubt related to differences 

 in chemical composition, as is shown by the fact that while a 

 gathering from one of the earlier falls {i. e. of higher specific 

 gravity) gave only 6-52 per cent, of water on analysis, that from 

 a later fall gave 7*8 per cent. The exact specific gravity of the 

 gathering chosen for analysis was not determined ; it gave the 

 following : — 



99-78 



The state of oxidation as it exists in the molecule was not 

 determined for want of material, but it was noticed that, with 

 ignition, the mica turned a deep pinchbeck brown colour, and 

 after losing weight, as the water was expelled, sensibly gained by 

 the absorption of oxygen. It is possible, therefore, that, as in 

 biotite, most of the iron is present in the ferrous state, and the 

 most plausible formula we can frame will represent that of a 

 mineral like biotite, in which three of the rays of the silica ring 

 have the composition — 



= Si<^>Al-0-H, 



and the other three — 



/O— Mn— 0— H 



= Si^ 



\0— Fe— 0— H 



two of the hydrogen atoms being replaced by sodium. This 



