Joly — On the Minerals of the Dublin and Wicklow Granite. 57 



give the whole nebula a symmetrical shape as crystallization pro- 

 gresses. This is symmetrical inclusion. The inclusions may he 

 mixed throughout the crystal in such abundance as to relegate the 

 parent crystal to fill the role of a form-producing paste only ; such 

 inclusions might form from the magma as the growth of the parent 

 crystal progressed. 



But such of these phenomena as are applicable to the present 

 case would surely be accompanied by confirmatory optical pheno- 

 mena. Will they again serve to explain the simultaneous stoppage 

 of growth of felspar and beryl ? — those large patches of white ortho- 

 clase visible over the surface of the hexagons, but perfectly smooth 

 and flush with the prism faces. How did the hexagonal virtue 

 extend its influence to the centre of those areas of the monoclinic 

 mineral? Within, in the cavities, the felspar crystals suggest an 

 independent growth — a growth independent of the hexagonal virtue 

 of their matrix. Had the hexagons ceased growing at that stage, 

 were abruptly-produced faces out of all relation with the symmetry 

 of orthoclase — necessarily so as the laminate crystals are oriented 

 in every direction — to be expected? Elsewhere in normal beryl 

 the felspar behaves after the general manner of inclusions — pro- 

 jects its solid angles out of the beryl, or, if the beryl be sufficiently 

 grown, is swallowed up. 



The distribution of the orthoclase in converging veins might 

 also be urged against the intercrystallization hypothesis ; but there 

 is a more direct argument forthcoming. 



It appeared that if the alteration hypothesis was correct, and 

 if the seat of the attack was to be sought for at the junction of 

 the prism with the orthoclastic matrix, then, in this region, con- 

 firmatory phenomena or the reverse might be expected. The con- 

 tinuity of prism and orthoclase has already been pointed out. It 

 appeared highly probable, on the alteration hypothesis, that this 

 junction was the seat of the reaction in the first instance. Sub- 

 jected to the influence of a potash felspar in a state of hot solu- 

 tion, the beryl was assailed and replaced, it may be at a very slow 

 rate. Such replacement may have been of the nature of alteration 

 merely, the berylium probably being removed, a re-arrangement 

 of the molecules occurring, and the crystalline net of orthoclase 

 replacing the original symmetry. 



On these grounds, however vague, I had a section cut from a 



SCIEN. PKOC, R.D.S. — YOL. V. PT. II. F 



