46 NAMING ROCKS ACCORDING TO THEIR GEOLOGICAL AGE. 



rocks are the same from the earliest times to the present. Other things 

 being equal, the older rocks are more altered; but as other things are not 

 equal, no abrupt line can be drawn at the tertiary age, as is now generally 

 done ; no characters exist whereby it can be done, and the line must remain 

 an arbitrary one. Alteration produces characters in the rocks that can be 

 used to indicate their greater age, or greater alteration — terms which are 

 not synonymous, although usually taken to be. The subject can, perhaps, 

 be best formulated as follows : all rocks upon the earth's surface undergo 

 alteration, and when exposed to the same conditions this is proportionate 

 to the age. It is the unquestioned duty of the petrographer to study these 

 changes, and starting from the least altered rock trace the continuous series 

 to the most altered one of that kind. Such a system of work has been 

 attempted here, so far as time and means have permitted. 



The presence or absence of fluidal cavities, which has been urged as a 

 distinction between tertiary and pre-tertiary rocks, seems to be related more 

 to depth, and the conditions to which the rocks have been exposed since 

 consolidation, than to age. The modern volcanic i-ocks are but the froth 

 of an eruption, compared with the massive eruptions that have taken place 

 in past time. The specimens collected are generally' of a surface nature, 

 and would allow the very ready escape of the inclosed vapors ; while at 

 some depth the escape could not take place as readily. Our older rocks 

 have in general suffered more denudation, and therefore are more likely 

 to contain fluid inclusions. Should it be shown that these fluid cavities are 

 in part, or entirely, of posterior formation to the rock, as has been urged 

 by Vogelsang,* and shown by Julien to be so in one case.t it would require 

 a new interpretation to be placed upon these and upon their occurrence. 

 My work would indicate that while part of the fluidal cavities are origi- 

 nal, some are posterior to the consolidation of the rock. A more fatal 

 objection to their use in separating tertiary from pre-tertiary rocks is the 

 finding of fluidal cavities in undoubted tertiary and post-tertiary rocks, t 

 Why quartz should be the mineral chosen to found this distinction upon, 

 and otiier minerals containing fluid inclusions in lavas should be ignored, 

 is a difficult thing to understand. 



The older rocks are, as a rule, entirely crystalline, a condition arising in 



* Pliilosopliie rler Gcologie, p. 155. 

 f Am. Quart. Microscopical Jour., 1S79, i. 103-115. 



J H. C. Sorby, Quart. Jour. Geol. Soc, 185S, xiv. -ISl ; Frauz Zirkel, Microscopical Petrography, vi. 

 112, 156, 157, 101, IGO, 167, 170, 265 ; Zeit. Deut. geol. GescIL, 1S68, xx. 117, 132. 



