ACID VOLCANIC ROCKS OF SOUTH MOUNTAIN. 829 



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 spheru- 

 litic 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 cryptocrystalline 

 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 pitch- 

 stones 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 Umstadt porphyries in Hessen. 



More recently Klockmann 4 (1890) describes the replace- 

 ment of the spherulitic crystallization in quartz-porphyries, 

 through secondary processes, by a fine grained aggregate of 

 quartz and feldspar. Osann 5 (1891) describes incipient devit- 

 rification 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 vol- 

 canics, and the possibility of devitrification and original simi- 

 larity is yet an open question in Germany. 



x Beitrage zur Petrographie der Plutonischen Gestein Abh. der Berliner Akad. 

 1869, p. 85. 



2 Mikroskopische Untersuchungen von Felsiten und Pechsteinen Sachsens T. M. 

 P. M., 1874, pp. 31-58. 



3Heinr. Otto Lang: Grundriss der Gesteinskunde, 1877, p. 43. 



4 F. Klockmann : Die Porphyre der Geol. d. s. g. Magdeburger unferandes m. 

 besonderes Beriisksichtigung d. auftretenden Eruptivgesteine Jahrbuch k. p. Geo. 

 Land. u. Bergakad. zu Berlin, 1890, vol. XL 



SZ. Geol. Ges. 691, 716. 



