60 Scientific Proceedings, Royal Dublin Society. 



pseudomorphous nature of that change is sufficiently accounted 

 for by supposing the reaction as engaging with the beryl only, 

 not with the tourmaline matrix. 



In speculating on the circumstances attendant on, and which 

 led to, the reaction, I may be pardoned perhaps for venturing to 

 suggest a theory of the formation of beryl and tourmaline through- 

 out the granite. 



The remarkably local nature of the distribution of the beryl in 

 the granite is well known. There are no crystals worth mention- 

 ing in the quarries situated close above those in which this abun- 

 dance of beryl is found. Rochetown Hill is mentioned by Weaver, 

 writing in 1819, as affording beautiful specimens. I searched the 

 quarries recently. The mineral is worked out. I found but one small 

 specimen. In a similar way the Killiney quarries have ceased to 

 yield ; they are now represented by Kingstown Pier, where speci- 

 mens may be found imbedded in the blocks used in its construc- 

 tion. At Ballybetagh a mere opening on the surface yielded a 

 group of crystals contained in a vein of porphyritic granite, which, 

 pursued further down, ceased to yield. The habitat of beryl is in 

 short the pocket or the vein, and, when the vein, generally close to 

 the surface. 



In all these respects it resembles that other accidental mineral 

 of the granite, tourmaline. 



Now it is most thinkable to suppose the rare elements glucinum 

 and boron originally diffused more or less uniformly throughout 

 the region, in which we will suppose the elements of granite to 

 be in a state of slow progressive crystallization in presence of 

 water. 



As cooling and solidification advanced, a concentration of those 

 elements would occur, which failed to take part in the molecular 

 arrangements going on throughout the magma, and pockets of 

 highly concentrated mother liquor would be formed. 



Many of these pockets, imprisoned at great depths, would re- 

 tain their position till loss of heat enabled, first, beryl, and then 

 tourmaline, to crystallize out. 



Many of these pockets again, as solidification advanced, may 

 be conceived as pressed out, and uniting in one outflow, forcing 

 their way to the surface in cracks left by the shrinking rock; 

 only crystallizing when from loss of pressure, or by conductivity to 



