ACID VOLCANIC ROCKS OF SOUTH MOUNTAIN. 827 



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-Ter- 

 tiary 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 orig- 

 inal 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 speci- 

 fied. 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. 



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 consid- 

 ers that in no very distant future the separation of effusine rocks into an older and 

 a younger series will prove untenable. 



