56 Scientific Proceedings, Royal Dublin Society. 



along the axis of elasticity. Their colour is dark green to yellow. 

 They are probably an alteration mineral, occurring principally 

 near the surface of the hexagons. To this constituent the dull 

 green colour of the crystals seems to be due. A little pyrites and 

 hematite are also present. 



The question that now presents itself for consideration is 

 this : — Here, in the field of the microscope, are two minerals, both 

 in the crystalline state — one true to the external hexagonal form in 

 molecular arrangement, while apparently separated into innumer- 

 able isolated portions by the second substance, which, in its optical 

 behaviour, shows no sympathy with the planes which limit in 

 common the extension of both minerals. From our knowledge of 

 its nature, it would indeed be altogether anomalous that it should 

 show such sympathy. 



Are we to suppose that we are here dealing with a crystal of 

 beryl which has been eaten into and replaced, at some period of 

 its history, by orthoclase, or with the result of simultaneous inter- 

 crystallization of beryl and orthoclase in the first instance ? 



In favour of this last hypothesis it is to be observed that it is 

 evidently quite unnecessary to suppose isolation of the beryl really 

 to exist, as unnecessary (and indeed obviously more so) as to sup- 

 pose, when looking at a map, that there was no connexion between 

 the patches of land islanded by the seas. In addition to which, in 

 consideration of the evident harmony of orientation of the beryl 

 molecules '.throughout, it is unthinkable. As, then, continuity of 

 the hexagonal matrix is in this crystal assured, are we to regard 

 the orthoclase as an inclusion merely — that the clustering laminae 

 and veins of felspar were formed progressively with the beryl, 

 although no crystallographic relation between the two bodies is 

 visible, or to be expected — that the phenomenon was due to the 

 compelling poiver or hexagonal virtue of the beryl ? 



Now this compelling power is generally effective in a different 

 way, or to much less extent. It may, indeed, force an abnormal 

 symmetry in a very partial degree on a body crystallizing in juxta- 

 position : cases of this are known. It may more commonly com- 

 pel into order the molecular confusion outside the parent crystal : 

 this may be merely growth, or it may give rise to an envelope of 

 smaller crystals of the same species as the parent crystal. It may 

 exert itself by taking up a cloud of fragments already formed, and 



