CHAPTER IV. 
PETROGRAPHICAL DESCRIPTION OF THE ACID ERUPTIVES. 
NOMENCLATURE. 
The question of the nomenclature of the acid volcauics has proved 
to be one of considerable interest and importance. In the discussion 
of suitable terms for the description of these rocks, it will be necessary 
to anticipate, in some degree, the results of their microscopic study. 
A variety of names have been applied by petrographers to the acid 
type of the older volcanic rocks. Under the general group of quartz- 
porphyries, Rosenbusch classifies them as microgranites, with a micro- 
granitic groundmass; granophyres, with a micropegmatic groundmass; 
felsophyres, with a microfelsitic base; and vitrophyres (including pitch- 
stones and pitchstone-porphyries), with a vitreous base. Fouque and 
Levy employ micro granitite, micropegmatite, and porphyre petrosiliceux 
as corresponding terms. British petrographers have described these 
acid rocks under the terms Jiornstoxes, claystones and clay stone-por- 
phyries, felsites, quartz- felsites, and felsite-poryhyries, agreeing in this 
respect with the older German usage, when they have not followed 
Eosenbusch. In America both German and English usages have been 
followed, with more or less confusing results. In the nomenclature of 
the South Mountain rocks an effort has been made to avoid such con- 
fusion and to use such a term or terms as shall accurately characterize 
them and all similar rocks. 
Among the acid eruptives of South Mountain are typical represen- 
tations of Bosenbusch's quartz-porphyries. Closely associated with 
them and impossible of separation by any sharp line of demarcation are 
acid rocks with every structural characteristic of modern lavas. 
Although possessing some characteristics in common with the felso- 
pliyres, these ancient prototypes of the rhyolite can not be included 
under that term, since they have a holociystalline groundmass. Inso- 
much as many of the English felsites have been shown by liutley, 
All port, Cole, and Bonney to be devitrified obsidians and pitch- 
stones, and thus, like the American rocks, representatives of the glassy 
lavas of pre-Tertiary times, these South Mountain lavas might, with 
some propriety, be termed felsites. Strict consistency would then coin- 
pel the replacement of the term quartz-porphyry by the more limited 
and less well-established term quartz-felsite, which was felt to be a dis- 
advantage. Moreover, the distinction between the rhyolitic lavas of 
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