308 



Silex, 33.64 



Alumina, 10.64 



Protoxide of iron, 8.83 



Magnesia, 34.95 



Water, 12.40 



Oxide of chrome, a trace. 



100.46 

 This analysis was twice repeated by Mr. Alex. MacDon- 

 nell, who obtained numbers almost identical with those above 

 given, and which it is, therefore, unnecessary to adduce. 

 These numbers, it may be easily shown, correspond very ex- 

 actly with the empirical formula, — 



Si0 3 + 3Al 2 3 4-29RO+2lHO; 



and the atoms may be so arranged as to give the rational for- 

 mula, — 



3 (Al 2 3 , Si 3 ) + 8 (3 R O, Si0 3 , 2HO)+5(RO, HO). 

 Pennine, it was subsequently found, had been previously 

 twice analyzed, first by Schweitzer, whose results do not dif- 

 fer very widely from those just given, and afterwards by Ma- 

 rignac and Descloiseaux, who are generally considered to have 

 accurately fixed its constitution. They give the following as 

 its empirical formula, viz. : — 



5 Si 3 + 2 Al 2 3 + 12 K O + 10 HO, 



which, as is obvious, is utterly irreconcileable with the analy- 

 sis just brought under the notice of the Academy. As respects 

 the cause of these discrepancies, the conjecture may be ha- 

 zarded, that they are due to the presence of intermixed por- 

 tions of other minerals. The specimens, for example, of 

 Pennine, brought to Dublin by Professor Jellett, are (some of 

 them) intersected by threads of talc, and incrusted with mi- 

 nute grossular garnets in perfect dodecahedral crystals. 



The crystalline system of Pennine is usually set down as 

 the third, the most common crystal being an acute rhombo- 

 hedron, whose apices are deeply truncated. The specimen, 



