A. W. Jackson — Nomenclature of Crystalline RocJcs. 119 



andesite; ai ; rora augite andesite; orthoclase 



porphyrite from trachyte; and diabase from dolerite. 



Those who hold to this distinction do not indeed deny the 

 frequent mineralogical identity of the io-U thu> m parated : but 

 they hold that it is desirable to express thus the difference in age 

 of the same mineral aggregate. With Prof. J. D. Dana * I 

 must confess myself unable to see the value of this distinction 

 for the nomenclature of the science. It appears to me much 

 more rational to call a certain mineral aggregate " quartz por- 

 phyry" or "liparite" (use which name one will) and to recog- 

 oize the fact that the rock thus named is sometimes of Silurian, 

 sometimes of Carboniferous, sometimes of Permian, and some- 

 times of Post-Cretaceous age. We find no difficulty in mineral- 

 arte porj " 



liparite, or rhyolite) under one species, in spite of the fact 

 that geological periods separated the dates of their formation. 

 One who would advocal 



be properly ignored and yet the geologist makes a similar un- 

 necessary and unphiloa q daily. 



Moreover those who insist upon this distinction are not con- 

 sistent. They separate quartz porplr r\ from liparite, but they 

 fail to separ : kite of the rlarz Mts. from the 



Mesozoic granite of Cornwall ; and they fail to separate certain 

 Triassic quartz porphyries of Germany from Silurian quartz 

 ' ies of England. In other words, the line is drawn 

 Mesozoic and Cenozoic but not betweeen Mesozoic 

 and Paleozoic. The reason for this (Roth, Beitrage zur Petro- 

 der plutonischen Gesteine in den Abhandlungei 



porphj 



e der Wissenschaften zu Berlin, 1869, p. 74) 

 "' e fact that the earlier geologists were led, through 

 of the Mesozoic rocks of Central Europe, to the 



conception that a period of comparative quiet separated the 

 widespread igneous activity of the Azoic and Paleozoic ages 

 from what appeared to be a "renewal . u this activity in a different 

 form in Tertiary and Post-Tertiary times. It appeared as though 

 the nature as well as the products of this renewed activity were 

 different. The pronounced absence of volcanoes, lava streams, 

 and tufas seemed to indicate that Pre-Tertiary activity was 

 subterranean while recent activity finds expression'^ (he surface in 

 tl)'' form of volcanoes, lava streams, tufa eruptions, and such 

 tremendous sheet overflows as those of Utah, and of Washing- 

 t"'i Ten-itorv. Oregon, and Northern California. 



The results of more recent and more w,de!v extended re- 

 searches do not seem to favor this idea. Paleozoic volcanoes, 

 lava streams, and even tufas have been found and we should 

 * J. D. Dana. On some points in Lithology. This Journal, iii, p. 336. 



