oDEU Til. PKISMATOIUAL GARNET. 



crystals from Spain, the substance of which is 

 less homogeneous, is between 3-3 and 3-4. 

 Compound Varieties. Twin-crystals. 1. Face 

 of composition parallel, axis of revojution perpen- 

 dicular to a face of f Pr = 90; 2. Face of com- 

 position parallel, axis of revolution perpendicular 

 to a face of P. .The individuals in both cases are 

 continued beyond the face of composition, and pro- 

 duce cruciform groupes, the crystals crossing each 

 other in the first case at right angle?, in the second 

 at angles of 60 and 120. By the addition of a 

 third individual to the latter, groupes resembling 

 stars, with six radii are forms. There are examples 

 of composition, parallel to both kinds of faces at once. 



OBSERVATIONS. 



1. According to an analysis by VAUQCELIK, of the va- 

 riety from Brittany, and another by KLAPROTH, of the 

 variety from St Gothard, prismatoidal Garnet consists of 



Silica 33-00 37*50. 



Alumina 44-00 41-00. 



Lime 3-84 0-00. 



Magnesia 0-00 0-50. 



Oxide of Iron 13-00 18-25. 



Oxide of Manganese 1-Ofr 0-50. 



It assumes a dark colour before the blowpipe, but does not 

 melt. 



2. This species occurs in imbedded crystals in primitive 

 rocks, particularly in mica-slate, in simple and compound 

 crystals, accompanied by prismatic Disthene-spar, dodeca- 

 hedral Garnet, &c. 



3. Simple crystals occur in St Gothard in Switzerland, 

 and the Greiner Mountain in Zillerthal in the Tyrol, some- 

 times curiously aggregated with crystals of prismatic 



