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THE JOURNAL OF GEOLOGY.

composition and structure to certain igneous rocks, but derived by metamorphism from something else."[1] Epi signifies the production of one mineral out of and upon 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 devitrification (Entglasung) 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, apobsidian, 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 holocrystalline groundmass should be termed quartz-porphyries, whether ancient or modern lavas. The writer realizes that the introduction 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 advocates 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.
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