Page:The Journal of geology (IA journalofgeology11893univers).pdf/413

incline to the opinion that the groundmass of certain quartz-prophyries is the result of the devitrification of a glassy lava. The late Dr. K. A. Lossen^[1] (1869), on comparing the spherulitic porphyries of the Harz Mountains with the obsidians of Lipari, Mexico and Java, found the resemblance sufficiently striking to lead him to declare that "the porphyry groundmass was originally crystallized as glass, and became crypotcrystalline through molecular rearrangement." Later, Kalkowsky^[2] (1874) suggests devitrification through the chemical activity of water, as the process by which the microfelsitic base of certain pitchstones and felsites was developed, and still later H. Otto Lang^[3] (1877) described a macroscopically unindividualized base which is similar microscopically to the devitrified base described by Kalkowsky. Sauer (1889) considers the Dobritz porphyries as the final alteration product of a pitchstone. C. Vogel comes to the same conclusion as to the Umstädt porphyries in Hessen.

More recently Klockmann^[4] (1890) describes the replacement of the spherulitic crystallization in quartz-porphyries, through secondary processes, by a fine grained aggregate of quartz and feldspar. Osann^[5] (1891) describes incipient devitrification in perlite and other glassy rocks from Cabo de Gata. Finally, Link (1892) considers that it is not impossible that the fine grained groundmass of some rocks from America that are closely related to mica-syenite-porphyries, was once glassy or at least partially glassy. Many no less capable observers still hold to an original difference between ancient and recent acid volcanics, and the possibility of devitrification and original similarity is yet an open question in Germany.