38 VOLCANIC ROCKS OP SOUTH MOUNTAIN. [bull. 136. 
which the rock has undergone subsequent to its solidification. If at 
the time of its solidification the rock presented the features of a 
rhyolite, as is believed to have been the case with much of the South 
Mountain acid lava, but since that time has become holocrystalline, 
both these facts — its original character and its present character — 
should be recognized in the name. It is believed that this result may- 
be secured by the retention of such well-established names as rhyolite, 
obsidian, trachyte, etc., preceded by a prefix which shall have such a 
signification as will indicate the altered character of the rock. The 
prepositions meta, cpi, and apo all indicate, as prefixes, some sort of an 
alteration. Their exact force has been thus defined by Professor Gil- 
dersleeve. Meta indicates change of any sort, the nature of the 
change not specified. This accords with the use of the prefix by Dana 
in such terms as "metadiorite" and " metadiabase." These terms have 
been recently revived to designate " rocks now similar in mineralogical 
composition and structure to certain igneous rocks, but derived by 
metamorphism from something else." 1 
Upi 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 epi- 
diorite, epigenetic hornblende, and epistilbite. Apo may properly be 
used to indicate the derivation of one rock from another by some spe- 
cific alteration. 
If, therefore, we decide to employ this prefix to indicate the specific 
alteration known as devitrification (Entglasung) we may obtain, by com- 
pounding it with the names of the corresponding glassy rocks, a set of 
useful and thoroughly descriptive terms like aporhyolite, apoperlite, 
apobsidian, etc., as to the exact meaning of which there can be no doubt. 
In accordance with this usage, it is proposed in this paper to call all 
the acid volcanic rocks the structures of which prove them to have once 
been glassy, aporhyolites, while such as were originally noncrystalline, 
or whose original character is in doubt, will be termed quartz-porphyries. 
The writer feels that the introduction of a new name into petrograph- 
ical literature 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 alteration has occurred. It is 
thought that aporhyolite and the suggested series of similarly formed 
terms meet these requirements. They are therefore adopted as prefer- 
able to any in present use. 2 
1 Whitman Cross, "On a series of peculiar schists near Salida, Colo.: Proc. Colo. Sci. Soc, Jan., 
1893, p. G. 
'-Since the ahove was written the term eorhyolite has heen proposed hy Dr. Nordenskjiild to cover 
ancient acid volcanics identical with those of the South Mountain. In a review of Dr. Nordenskjold's 
able paper, "Ueber archseisehe Ergussgesteine aus Smaland " (Bull. Geol. Instit. Upsala No. 2, vol. 1, 
1893), the writer has discussed the disadvantages that attend the use of any term or series of terms 
which carry with them the idea of age. The reader is referred to this review, which appeared in the 
American Geologist for March, 1896, pp. 179-184, for a fuller statement of the writer's idea of the rela- 
tion existing between devitrification and age of volcanics. 
