828 THE JOURNAL OF GEOLOGY. 



composition and structure to certain igneous rocks, but derived 

 by metamorphism from something else." 1 Epi signifies the pro- 

 duction of one mineral out of and upo?i another. This prefix has 

 not been much used. We find it in such terms as epidiorite, 

 epigenetic hornblende . and epistilbite. Apo may properly be 

 used to indicate the derivation of one rock from another by some 

 specific alteration. 



If, therefore, we decide to employ this prefix to indicate the 

 specific alteration known as devitrifiction [Entglasuug) we may 

 obtain, by compounding it with the name of the corresponding 

 glassy rocks, a set of useful and thoroughly descriptive terms, 

 like aporhyolite, apoperlite, apobsidia?i, etc., as to whose exact 

 meaning there can be no doubt. In accordance with this usage 

 it is proposed to call all the acid volcanic rocks, whose structures 

 prove them to have once been glassy, aporhyolites. While those 

 which have consolidated at a sufficient depth to secure a non- 

 crystalline groundmass should be termed quartz-porphyries, whether 

 ancient or modern lavas. The writer realizes that the introduc- 

 tion of a new name into petrographical nomenclature is to be 

 deplored unless it can be shown that the name is formulated in 

 accordance with certain well defined principles. A good rock 

 name should express composition, original structure, and, as far 

 as possible, the process of alteration, if any, that the rock has 

 undergone. It is thought that aporhyolite and the suggested 

 series of similarly formed terms meet these requirements. They 

 are, therefore, adopted as preferable to any in present use. 



Paleozoic and pre-Paleozoic acid volcanics have long been 

 studied on the Continent, Although their variation from the 

 modern type of acid volcanic, 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 advo- 

 cates of devitrification and of an original glassy base for the 

 ancient lavas. R. Ludwig (1861), and Vogalsang 2 (1867) 



1 Whitman Cross: On a Series of Peculiar Schists near Salida, Colorado. Proc. 

 Col. Sc. Soc, 1893, p. 6. 



2 Philos. d. Geologie, 144, 153, 194. 



