300 



Orenville A. J. Cole — Rhyolites of the Vosges. 



other, and aid doubtless in bringing about the alteration of the 

 spherulites. It is to this admixture of glass and this slight differentia- 

 tion from it that I would prefer to ascribe the absence of double 

 refraction in so many incipient spherulites of modern date, rather 

 than adopt the view held by some petrographers that such bodies 

 may largely consist of opal. 



Delesse, with his usual perception, detected in the matrix of the 

 rock of Wuenheim the perlitic jointing, the " structure fendillee et 

 globuleuse entrelacee," explaining it as a product of contraction when 

 the mass had become practically solid. Microscopic sections would 

 have revealed it to him in exceptional completeness, and the former 

 glassy condition of these dull felsitic rocks may be considered as 

 proved by the presence of the structure in such perfection. The 

 spherulites, whether grey or red, are alike brown in section, and 

 resemble the well-known type, the "felsospherites " of Gei-man 

 authors; while their minute globulitic constitution seems to be still 

 retained, despite the secondary granulation visible under polarised 



Fig. 1. — Section of rock of Tolcsva, showing disturbance of the lines of flowhetween. 



the components of the large irregular spherulites. x 80. 

 Fig. 2. — Section of altered perlitic lava of Wuenheim, showing rays belonging to 



two large "skeleton-spherulites." x 18. 



light. Vogelsang,^ indeed, comments on the " surprising resem- 

 blance " between the structures of the rock of Wuenheim and certain 

 Hungarian lavas ; and it is interesting to note that Faujas de St.- 

 Fond ^ described, eighty years ago, the similar pyromeride of Corsica 

 under the name of Roche porphyroide rather than porphyre, stating 

 his belief that the constituents had consolidated more rapidly than is 

 the case in the latter class of rocks. 



The most remarkable rock from the "globular" series of Wuenheim 

 is one of limited occurrence, in which the spherulites, about 1 centi- 

 metre in diameter, are closely crowded together, apparently to the 

 almost complete exclusion of the matrix. Instead of being compact 

 and homogeneous, these spherulites are seen on broken surfaces to 

 be composed of alternating red and lighter rays, suggesting the 



^ Die Krystalliten, p, 168. "^ Essai de Geologic, tome ii. p. 245. 



