Joly — On the Minerals of the Dublin and Wicklow Granite. 59 



It will be noticed in this section that tourmaline is present in 

 tufts and dark masses encroaching on the edge b, and generally 

 mixed through the felspar. Elsewhere it is conspicuously of 

 secondary origin to beryl, and my crystal penetrated by tour- 

 maline, before-noticed, is explained by alteration of a similar 

 character to this. 



It is to me inconceivable that this jumble of fragments of 

 beryl, with molecular orientation in every direction, scattered 

 through a sea of felspar, owes its external hexagonal form to the 

 hexagonal virtue of the beryl. If, in short, the beryl was not 

 able to keep itself in order, how, on the intercrystallization 

 hypothesis, was it able to shape into order, against their normal 

 molecular tendencies, the molecules of felspar ? 



What were the nature and circumstances of the reaction which 

 led to this alteration or substitution ? Was it hydro-igneous or 

 simply igneous ? 



It seems probable, in the first place, that intermixtures like 

 this of bodies of very different melting points is most readily ex- 

 plained by hydro-igneous formation of one or both the bodies. 

 Thus Daubre, by attack with steam at 400° 0., obtained crystals 

 of quartz and pyroxene imbedded in an easily fusible matrix, 

 derived from the glass tubes employed. 



Other arguments for low temperature origin of the felspar 

 exist. Thus we find a beryl moulded round by felspar: the edges 

 of the beryl are sharp and well defined, although its melting point 

 is far below that of orthoclase. 



Again we find the beryl coloured yellow, green, or blue, but it 

 loses all colour, according to experiment, at 350° C, after an hour's 

 heating. 



There is internal evidence too. Examined with high powers 

 the sections reveal innumerable enclosures. Some glass, but some 

 composed of liquid, with movable gas bubble. These are plenti- 

 ful, both in beryl and felspar. In places they range in veins and 

 strings, resembling fluxion structure. Tiny crystals (?) accom- 

 pany in shoals. With inclined microscope the gas bubbles may 

 often be induced, on tapping the stage, to travel from end to end 

 of the cavity. 



It seems probable, then, that the change experienced by these 

 beryls was effected at low temperature, or hydro-igneously. The 



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