56 MR. J. W. GEEGORT ON THE 



varioles are typically developed. I did not myself have the good 

 fortune to obtain from the area specimens of all the types of vario- 

 litic structure described by Prof. Rosenbusch from Berneck ; but 

 it is probable that a more careful examination, especially to the 

 north of the Oelschnitz, would show that they may be all grouped 

 into the two classes- — of those that are true contact-selvage-products 

 and those formed within the diabase. jS"o doubt occasionally a more 

 perfect variole may be found among the former, and a rudimentary 

 one among the latter, but as a rule this division will probably be 

 found to hold. 



IV. The " PSEUDOCRYSTALLITES." 



In the course of M. Michel-Levy's admirable description * of the 

 microscopic structure of the Mont Genevre variolite, he described 

 and discussed the nature of certain enclosures in the varioles which 

 he named " pseudocrystallites." In the previously published de- 

 scription of the fine plate f in the ' Mineralogie Micrographique/ 

 MM. Pouque and Michel-Levy attributed these bodies to " fissures 

 of retreat;" but in the latter author's more detailed subsequent 

 examination he was led by the peculiar optical properties of these 

 structures to regard them as felspathic pseudocrystallites. He 

 observed that when their long axes are parallel to the neighbouring 

 felspar fibres they extinguish at once throughout their entire length, 

 but when they traverse the radial fibres they disappear under 

 crossed nicols, as the felspars seem continuous across them ; hence 

 M. Michel-Levy was forced to regard such pseudocrystallites as 

 composed of a complex aggregate of felspar crystals, difi'erently 

 orientated, so that their long axes were in the same straight line as 

 those of the radiating felspar fibres of the variole. As it did not 

 seem easy to see how such an arrangement should have taken place 

 if the felspar in these bodies were a secondary product, M. Michel- 

 Levy was led to this modification of the earlier theory. 



In the paper by Mr. Cole and myself, we were led, by noticing 

 how the larger rectangular fissures passed off into the minute 

 branching irregular cracks, to regard them as due to rifts formed 

 during contraction, but we could offer no explanation of the abnor- 

 mal optical properties or arrangement of the felspathic constituents. 

 We concluded J that they were " little fissures due to fracture or 

 contraction," and that " until a similar structure is found in other 

 rocks, so that ample comparison may be made, the last word cannot 

 be said on these interesting ' pseudocrystallites.' " 



At Berneck there is a great development of these structures 

 throughout the whole rock ; they frequently occur in great numbers 

 in bands which pass through variolite and groundmass alike, while 



* " Memoire sur la variolite de la Durance," Bull. Sec. g6ol. France, 3™®ser. 

 vol. V. (1877), p. 233. 

 t PI. xxiv. fig. 2. 

 + Quart. Journ. Geol. Soc. vol. xlvi. (1890), pp. 313 & 314. 



