Page:The Journal of geology (IA journalofgeology11893univers).pdf/410

with the older German usage, when they have not followed Rosenbusch. In America both German and English usage has been followed with more or less confusing results. In the nomenclature of the South Mountain rocks an effort has been made to avoid such confusion and to use such a term or terms as shall accurately describe them and all similar rocks. No one of the terms mentioned succeed in doing this. Although, perhaps, most nearly like the felsophyres, these South Mountain rocks cannot be included under that term since they now possess a holocrystalline groundmass.

In so much as many of the English felsites have been shown by Rutley, Allport, Cole, and Bonney to be devitrified obsidians and pitchstones, and thus, like these American rocks, the representatives of the glassy lavas of pre-Tertiary times, these pre-Cambrian lavas of the South Mountain might with some propriety be termed felsites. Felsites, however, though useful as a field name may well be objected to as an inaccurate petrographical term. It was originally used to describe an acid base, unresolvable to the naked eye, and at first supposed to be a single mineral.^[1] With the introduction of the microscope this macro "felsitic" base was resolved into the microgranitic, micropegmatitic, and microfelsitic groundmass, the point of ignorance being shifted from the felsitic base, macroscopically unresolvable to the microfelsitic base, which is microscopically unresolvable. On the continent felsite has been practically replaced by these terms. British and American petrographers have retained it as a field name for rocks formed of this macroscopically unresolvable base without phenocrysts or with inconspicuous phenocrysts. The South Mountain rocks are both without phenocrysts, with inconspicuous phenocrysts, and with abundant and conspicuous phenocrysts. As this irregular distribution of the porphyritical crystals may characterize a single lava flow, it does not seem a sufficient ground for a separation of rock types.