MINKRAI.DiilOAl, NCilKS— ANDKU'SdN. 9 



is «o, tu what cause can we attribute tlieir formation it not to solution V 

 An alternative hypothesis is that tlie faces of c (0001) and y*(l()ll) 

 occurring at different levels in this crystal and Nos. ix.-xi. jii-e-existed, 

 and are i-evealed when the overlying layers are dissolved away; that is, 

 they result from zonal growth. To test this hypothesis I made sevei-al 

 attempts to produce such an undei-lyiiig surface by splitting the crystals 

 in a direction paiallel to the basal plane, but in no case did I succeed in 

 obtaining a surface with well formed faces of r and p, even wlien there 

 was strong evidence that a pronounced structure plane extended across 

 the crystal in this direction ; the fracture was invariably rough (»• 

 subconchoidal. A second hypothesis which might be put forward is that 

 these ciystals consist of a nuinber of sub-individuals in parallel position 

 in which zonal structure is strongly developed, and the hexagonal hillocks 

 ai'e really forms of accretion which are now being dissolved mainl^^ in a 

 lateral direction, giving rise to concentric hexagonal markings and 

 elevations. But it is difficult to imagine tliat an appearance such as 

 that presented by crystal No. x. (PI. i., fig. 3) ctmld be thus produced. 



If w^e assume that the pits and hillocks on these crystals result from 

 solution, not from gi-owth, we have to find a plausible explanation of the 

 fact that the etch figures are in general so large, so well defined, and 

 bounded by ti-ue, not vicinal faces, as we should expect. Our knowledge 

 of etch figures is now extensive, but much of it has been obtained by 

 study of the results of artificial etching, and it is possible that natural 

 etch figures, wliiclv are in most cases produced by the long continued 

 action of very dilute solvents, may present features which cannot be 

 reproduced in the laboratoiy, where solvent action is, as a rule, rapid. 

 One of the latest writers on etch figures'' holds that the point of origin 

 and the distribution of etch figures on a crystal face have a close relation 

 to the molecular sti'ucture, that, in fact, faces and directions of maximum 

 cohesion are attacked much more slowly than those in which the cohesion 

 is of a lower degree. This is in accordance with the observed fact, well 

 illustrated in these beryl crystals (PI. ii., figs. 4, 6, 7), that the pits are 

 disposed in more or less straight lines which ai'e presumabl}- lines of weak 

 cohesion. The occurrence of stepped pits and stepped hillocks, which are 

 characteristic of the ciystals, is readily explicable on this theory, as the 

 teri-acing is determined by the presence of cleavage planes and possibly 

 also by zonal growth planes. 



As regards the figure faces, McNairn has found that these are not 

 truly curved, but are made up of a large series of small planes inclined 

 at low angles to one another, and that they i-epreseut as close an 

 approximation to true crystal faces as is possible under the circum- 

 stances'^. Following Goldschmidt, he considers that " in pit formation 

 there are two opposing tendencies, one of which would result in the 

 formation of round excavations if unopposed, and the other in the 

 development of true crystal faces. Thus the net result is these faces 



' McNaiin— Traus. Roy. Canad. lust., xi., 2, 1917, pp. 229-267. 

 8 McNairn — Loc. cil., p. 258. 



