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ACID VOLCANIC ROCKS OF SOUTH MOUNTAIN.

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It is very generally recognized that structural features are not conditioned by the geological age of rocks, but are, on the other hand, a function of the conditions of consolidation. That the conditions attending the consolidation of surface flows in pre-Tertiary times do not differ from those attending the consolidation of similar flows in post-Tertiary times has been illustrated by a wide survey of pre-Tertiary and Tertiary rocks on the part of Allport, Judd, Teall and others^[1] With this recognition has come the growing conviction among petrographers that mere age should be eliminated as a factor in rock nomenclature.^[2] While this is true, it is felt, on the other hand, that there should be some recognition in the rock name of the alteration 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 it is believed much of the South Mountain acid lava did, but since that time has become holocrystalline, both these facts, its original character and its present alteration, should be recognized in the name.

Such a result might be secured by the retention of such well established names as rhyolite, obsidian, trachyte, etc., preceded by a prefix which shall have such a designation as to indicate the altered character of the rock. The prepositions meta, epi and apo, as prefixes, all indicate some sort of an alteration. Their exact force has been thus defined by Professor Gildersleeve: 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

↑
- Allport: Address of the Pres. of the Geo. Sec. of the British A. A. A. S., 1873, and many other writings by the same author.
- Judd: On the Gabbros, Dolerites and Basalts of Tertiary Age in Scotland and Ireland.Q. J. G. S., Vol. XLII., 1886, pp.49-97.
- Teall: British Petrography, pp. 64-69.
↑ Reyer, Tietze, Reiser, Reusch (H. H.), and Suess support the statement that age is not a just ground of distinction between eruptive rocks, and Rosenbusch considers that in no very distant future the separation of effusine rocks into an older and younger series will prove untenable.

[1] 

Allport: Address of the Pres. of the Geo. Sec. of the British A. A. A. S., 1873, and many other writings by the same author.

Judd: On the Gabbros, Dolerites and Basalts of Tertiary Age in Scotland and Ireland.Q. J. G. S., Vol. XLII., 1886, pp.49-97.

Teall: British Petrography, pp. 64-69.

[2] Allport: Address of the Pres. of the Geo. Sec. of the British A. A. A. S., 1873, and many other writings by the same author.

[3] Judd: On the Gabbros, Dolerites and Basalts of Tertiary Age in Scotland and Ireland.Q. J. G. S., Vol. XLII., 1886, pp.49-97.

[4] Teall: British Petrography, pp. 64-69.

[2] Reyer, Tietze, Reiser, Reusch (H. H.), and Suess support the statement that age is not a just ground of distinction between eruptive rocks, and Rosenbusch considers that in no very distant future the separation of effusine rocks into an older and younger series will prove untenable.

[1]

[2]