Page:The Journal of geology (IA journalofgeology11893univers).pdf/414

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 hälleflinta 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 Nordenskjöld.^[2] In Belgium Vallée-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 volcanics has been recognized and emphasized, and the idea of devitrification 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 process which has been forming felsites out of rhyolites. What he says is of interest in its anticipation of ideas now more generally accepted. "This devitrification gives rise in the older and more altered rhyolites to the feldspar, quartz and microfelsitic