628 Scientific Proceedings, Royal Dublin Society. 



Its mode of crystallization is unusual, being almost invariably 

 radiated, though the crystals are hexagonal prisms. 



In Mr. Joly's Paper it is shown that through some of the beryl 

 crystals there were veins of felspar, and that near the base of a 

 hexagonal prism there was an admixture of orthoclase and beryl. 

 We may arrive at an approximation of the quantity of orthoclase 

 admixed with the beryls which were the subject of the foregoing 

 analyses, by taking into account the alkalies present, and assuming 

 that they were not a part of the beryl, but constituents of the 

 orthoclase ; and, further, by taking into account the alumina pre- 

 sent in beryls and in orthoclase. 



Out of seventy-eight analyses of different specimens of ortho- 

 clase, the following numbers were taken as representing its average 

 composition : — 



Si0 2 65 per cent. A1 2 3 19 per cent. Alkalies, 14 per cent. 



The alkalies in beryl are 1*08 per cent., corresponding to a 

 maximum of orthoclase of 7'7 per cent. While the alumina in the 

 Grlencullen beryls is 32 "5 per cent., that in twenty other specimens 

 is 18 per cent., or the same proportion as that in orthoclase. The 

 difference in the proportion of silica in orthoclase and beryls is not 

 great; in the latter mineral it amounts to 67 per cent., or 2 per 

 cent, more than in orthoclase. 



After the elimination of such quantities of bases and silica as 

 may constitute an admixture of other minerals, we have to account 

 for the presence of only 47*47 per cent, of silica, and the large pro- 

 portion of 32 "49 per cent, of alumina, that is to say, a deficiency of 

 21 per cent, of the former, and an excess of 16 per cent, of the 

 latter ; and the only way in which this can be done is by assign- 

 ing to the beryls of Grlencullen a formula which represents their 

 constitution in the manner already described. 



