494 Scientific Proceedings^ Royal Duhlni Society. 



Microscopical Characters. 



In thin section the mineral still appears compact when viewed with a 

 pocket lens. With a magnification of seventy diameters it exhibits a 

 granular structure, and appears to have higher refraction than quartz. 

 Between crossed nicols it is seen to be birefringent, the field being finely 

 speckled. A higher power shows that in places the structure consists of 

 rough radial aggregates, in which the components may occasionally be seen 

 to have straight extinction. 



Identification. 



Comparing the analysis with column II of the table, it will be seen that 

 there is good agreement with an average muscovite. The latter, however, is 

 not affected by acids, whereas the Ballycorus mineral is completely decom- 

 posed by hot concentrated sulphuric acid. 



The term Finite is applied to a number of compact substances, which are 

 generally alteration products, and have a composition similar to that of 

 muscovite. Most, if not all, of them difier from muscovite, and agree with 

 our mineral, in being decomposable by sulphuric acid. As was to be expected, 

 since they are generally impure substances, none of the named varieties was 

 found to agree in all points with the Ballycorus mineral, though many come 

 near it. 



The best agreement seems to be with pinitoid. That mineral, according 

 to A. Knop,^ is a basic silicate, mica-like in composition, containing water, 

 decomposable by hot sulphuric acid, micro to cryptocrystalline, of usually 

 earthy to dense compact habit, of a leek-, oil-, or greyish-green to whitish 

 colour, which may pass over into various shades of red. Specific gravity, 2788 ; 

 H, 2'5. It is a secondary mineral formed by hydration, which frequently 

 occurs as pseudomorphs after felspar in altered porphyry. 



So far this might apply to the Ballycorus mineral, except that it was not 

 observed in shades of red. Knop's analysis would account for this, and 

 reveals a considerable difference in composition, for the pinitoid analysed 

 gave 8'84 per cent, oxide of iron against 0'24 per cent, in our mineral. 

 Pinitoid, too, is low in potash, 5-85, as compared with 9-00. 



Column III of the table is the mean of the twelve piuite analyses 

 given by Dana.- Comparison with column I shows the close agreement. As 

 a matter of fact, all the figures for the Ballycorus mineral are inside the 

 extremes occurring in those analyses. 



' " Neues Jahrbuch fiir Mineralogie," etc., p. 558. 1859. 

 - Loc. cit. , p. 622. 



