bascom.] APORHYOLITES. 59 
Opinions of petrographers. — Paleozoic and pre-Paleozoic acid vol- 
canics have long been studied on the European continent. Although 
their variation from the modern type of acid volcanics, rather than their 
resemblance to that type, has, for the most part, been emphasized by 
German and French petrographers, there have not been wanting able 
advocates of devitrification and of an original glassy base for the 
ancient lavas. 
11. Ludwig' (1861) and Vogelsang 2 (1867) inclined to the opinion that 
the ground mass of certain quartz-porphyries is the result of the devit- 
rification of a glassy lava. 
The late Dr. K. S. Lossen 3 (1869), on comparing the spherulitic por- 
phyries 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 cryptocrystalliue through molecular rearrangement. Later, 
Kalkowsky 4 (1874) suggested that devitrification through the chem- 
ical activity of water was the process by which the microfelsitic base of 
certain pitchstones and'felsites was developed; and still later, H. Otto 
Lang 5 (1877) described a macroscopically unindividualized base which 
is similar macroscopically to the devitrified base described by Kalkow- 
sky. Sauer ° (1889) considered the Dobritz porphyries as the final altera- 
tion product of a pitchstone. More recently Klockmann 7 (1890) has 
described the replacement of the spherulitic crystallization in quartz 
porphyries, through secondary processes, by a fine-grained aggregate 
of quartz and feldspar. 
Osann 8 (1891) described incipient devitrification in perlite and other 
glassy rocks from Cabo de Gata. Finally, Link (1892) considered that 
the fine-grained groundmass of some American rocks closely related 
to mica-syenite-porphyries was once glassy, or at least partially glassy, 
and 0. Vogel 9 (1892) reached the same conclusion as to the Umstadt 
porphyries in Hessen. Many no less capable observers still hold to 
an original difference between ancient and recent acid volcanics, and 
the possibility of devitrification and an original similarity is yet an 
open question in Germany. 
In France, La Croix 10 describes andesites from Martinique in which 
the glass has altered into quartz spheruhtes and a granular quartz 
aggregate. 
It is interesting to note that many of the hallefiinta of Sweden, 
>Erl. zur geol. Karte Hessens, Bl. Dieburg, 1861, p. 56. 
2Phil. do geologie, pp. 144, 153, 194. 
3 Beitrage zur Petrograpbio dor plutoniscben Oeateino: Abhandl. der Berliner Acad., 1869, p. 85. 
4 Tsehennaks mineral. Mittbeil, pp. 31, 58. 
f 'Grundri.s.s der Gesteinskunde, p. 43. 
6 Erl. zur geol. Spocialkarto Sacksens. Bl. Meissen, pp. 81-91. 
'Die Porpbyro; Dor geol. Auf ban sogen. Magdeburger U ferrandes mit besonderer Beriicksichti- 
gung der auftretenden Ernptivgesteine : Jabrbucb K., preuss. geol. Landesanstalt,Vol. XI. 
» Zeitsohr. Deutsch. geol. Gesell., Berlin, pp. 691, 716. 
9 Abhandl. geol. Landesanatalt von llosson, Vol. II, p. 38. 
10 Coruptes-rendus, CXI, p. 71. 
