1881 .] 
235 
[Merrill. 
felsitic base of the unaltered andesites, and of some allied rocks, 
with the alteration and devitrification products in the basalts, 
andesites, trachytes, rhyolites, felsites, etc.” The point to which 
attention is specially called, in this case, is the expression : 
“ original microfelsitic base of the unaltered andesites.” F. Zirkel 
introduced the term, microfelsitic , into the science, and finding it 
to be of somewhat difficult precise definition, he expressly stated 
where the microfelsitic base was typically developed in order to 
leave no doubt as to the sense in which he used the term. A 
translation of his words into English would read as follows : “ It 
is chiefly in rocks which are decidedly rich in silica , as the quartz 
porphyries, or more especially the liparites in which it is developed, 
etc.” (Zirkel, Mikroskop. Beschr. der Min. u. Gest. 1873. S. 281 ). 1 
Now, although the andesites are not basic in the sense of that 
term as applied to basalts and their ante-tertiary equivalents, 
melaphyr and olivin-diabase, yet, according to Mr. Wadsworth 
himself (page 280), they stand next to basalts in this respect 
among the tertiary and post-tertiary volcanic rocks. They are 
certainly quite far removed from rocks “ decidedly rich in silica, 
like the quartz-porphyries.” Moreover, the originator of the 
term, microfelsitic, in the very volume in which he introduces it, 
makes no mention of the development of that base in the section 
devoted to the andesites, excepting, in one instance of the discus- 
sion of certain hornblende-andesites, to say that it almost never 
occurs (Mikr. Beschr. S. 406), while throughout the sections de- 
voted to both hornblende-, and augite-andesites there is frequent 
mention of glassy base. 2 Again, in his introduction to the report 
1 Indeed, a most casual acquaintance with the history of studies upon the ground- 
mass of the quartz-porphyries will remind one that it was in connection with them 
that the term, microfelsite, was first used. (See Rosenbusch, Mikroskop. Phvs. der 
massig. Gest. 1877, S. 60-72; also, Zirkel, Mikroskop. Beschr. 1873, S. 265-281, 321-331; 
Neues Jahrb. fur Mineral. 1878, S. 514). 
It is not intended, here, to argue that the “microfelsitic ” substance does not occur 
in andesites, but only to notice the original sense in which the name was employed. 
2 Under hornblende-andesite (Mikroskop. Beschr. S. 405) Zirkel alludes to the ground- 
mass as being very similar to the trachytic, and refers to page 386 under trachytes. 
There we find the words: “ Die mikroskopische Structur der Grundinasse der Trachyte 
verhalt sich im Allgemeinen von derjenigeu derLiparite ziemlich verschieden, nament- 
lich ist die dort so verbreitete mihroj elsitische Substanz hier nur sehr sellen zur Entwicke- 
lung gelangt . ” 
