830 THE JOURNAL OF GEOLOGY. 



In France, La Croix 1 describes andesites from Martinique 

 in which the glass has altered into quartz spherulites and a 

 granular quartz aggregate. It is interesting to note that many 

 of the halleflinta of Sweden, which, like the South' Mountain 

 volcanics, were once described as sedimentary, are proving to be 

 acid volcanics preserving the features of their modern equivalents. 

 Quite recently, glassy and rhyolitic structures in these rocks 

 have been observed and described by Otto Nordenskjold. 2 In 

 Belgium Vallee-Poussin seems to be the only writer who has 

 brought out the resemblance between the eurites of that country 

 and modern rhyolites. He describes at some length structures 

 similar to those possessed by the aporhyolites of South Mountain. 

 A vacillating state of mind as to the matter of nomenclature is 

 indicated in the titles of his successive papers. 3 



In England the rhyolitic character of the ancient acid vol- 

 canics has been recognized and emphasized, and the idea of devi- 

 trification is widely accepted. Allport, Cole, Bonney, Rutley and 

 Harker have accomplished most valuable work along this line. 

 Dr. Wadsworth 4 was the first American petrographer to advocate 

 the abandonment of age as a factor in rock classification ; 

 while at the same time he recognized devitrification as the pro- 

 cess which has been forming felsites out of rhyolites. What he 

 says is of interest in its anticipation of ideas now more gener- 

 ally accepted. " This devitrification gives rise in the older and 

 more altered rhyolites to the feldspar, quartz and microfelsitic 



1 Comptes rendus, CXI., p. 71. 



2 Opus cit. 



3 Les Anciennes Rhyolities dites Eurites de Grand-Manil. Bull. Acad. R. de 

 Belg., 3d series, Tome 10, 1885, pp. 253-315. 



Les Eurites quartzeuses (rhyolite anciennes) de Nivelles et des Environs. Bull. 

 Acad. R. des Sc. etdes Beaux-Artes de Belg. 56 annue, 3d series, Tome 13, No. 5, 1887. 



4 M. E. Wadsworth : Notes on the Mineralogy and Petrography of Boston and 

 vicinity. Proc. Boston Soc. Nat. His., vol. XIX., May, 1877, p. 236. 



On the Classification of Rocks. Bull. Mus. Comp. Zool., Harvard College, vol. 

 V., No. 13, June, 1879, p. 277. 



