TARIOLITIC DIABASE OF THE EICHTELGEBIRGE, 57 



the individual pseiidocrystallites can often be traced from the one 

 into the other. There seems no reason to doubt the identity of 

 these bodies with those at Mont Genevre ; the difference between 

 them is only in the infilling material. At Eerneck the rock is 

 permeated with calcareous infiltration- products which line the 

 joints and form veins that cross the spheroids ; hence the pseudo- 

 crystallites are mainly filled with calcareous or dolomitic products : 

 but at Mont Genevre the veins and cracks are mainly occupied by 

 felspathic material, and hence these rifts often contain plagio- 

 clase fibres. At Berneck the secondary material in the spaces is 

 quite different from that which forms the rock on either side ; they 

 are therefore sharply marked off" from it in any condition of illu- 

 mination. At Mont Genevre, on the other hand, not only is the 

 material that has filled up the rifts similar to that of the surrounding 

 rock, but the felspar has been deposited in optical continuity with 

 the crystalline fibres that were broken and separated by the forma- 

 tion of the pseudocrystallite. Such cases of the restoration of the 

 optical continuity of crystals across cracks by the deposition of 

 secondary minerals in the latter appear to have been fully estab- 

 lished *. If this suggestion be accepted, it would supy)ly a very 

 striking and complex case of this phenomenon, while it would also 

 aff'ord a full explanation of the abnormal optical properties that 

 M. Michel-Levy has described in these structures. 



It appears to have been these optical characters almost entirely 

 that led M. Michel-Levy to his theory of the origin of the " pseudo- 

 crystallites " ; apart from these, and though in many cases ib must 

 be admitted that the regular reticulation of these structures does 

 present the aspect of a crystalline meshwork, the evidence in favour 

 of the origin of these bodies as cracks and fractures subsequent to 

 consolidation seems fairly conclusive. This view is supported by 

 their irregular distribution, both in the varioles and the groundmass, 

 well shown in specimens from Berneck, b}' the gradual transition 

 that can frequently be traced from them into undoubted cracks, 

 and by the fact that they sometimes pass from a variole into the 

 groundmass. The freshness of the material is an additional argu- 

 ment, and apparently this alone was sufficient to induce Frof. von 

 Giimbel to consider them as secondary. 



As first figured by MM. Fouque and Michel-Levy, there was a 

 certain regularity in the disposition of these cracks, as the}^ occurred 

 along a zone arranged concentrically with the variole ; in this case 

 the variole is sharply marked off from the groundmass, and no doubt 

 the shrinkage that caused this distinct separation found out a line 

 of weakness due to one of the concentric zones of glassy matter that 

 often occur in spherulites. Iddings t bas figured a spherulite in 

 which a number of trichites are disposed in a similar circle to that 

 in this variole. AVhere, however, the spherulite passes gradually 



* See, for example, the cases figured in this Journal by Miss Eaisin, toI. xlv. 

 p. 253, and hv Cole and Gregory, vol. xlvi. pi. siii. fig. 1. 



t J. P. Iddings "Obsidian Cliff;" 7th Ann. Eep. U.S. Geol. Surv., Wash- 

 ington, 1888, p. 276, pi. XV. fig. 4. 



