826 THE JOURNAL OF GEOLOGY. 



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 repre- 

 sentatives 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 petro- 

 graphical term. It was originally used to describe an acid base, 

 unresolvable to the naked eye, and at first supposed to be a sin- 

 gle mineral. 1 With the introduction of the microscope this 

 macro " felsitic " base was resolved into the microgranitic, micro- 

 pegmatitic, and microfelsitic groundmass, the point of ignorance 

 being shifted from the felsitic base, macroscopically unresolv- 

 able to the microfelsitic base, which is microscopically unresolva- 

 ble. On the continent felsite has been practically replaced by 

 these terms. British and American petographers have retained 

 it as a field name for rocks formed of this macroscopically unre- 

 solvable base without phenocrysts or with inconspicuous pheno- 

 crysts. The South Mountain rocks are both without phenocrysts, 

 with inconspicuous phenocrysts, and with abundant and conspic- 

 uous phenocrysts. As this irregular distribution of the porphy- 

 ritical crystals may characterize a single lava flow, it does not 

 seem a sufficient ground for a separation of rock types. 



1 Gerhard : Beitrage zur Geschichte des Weissteins des Felsit und anderer 

 verwandten Arten" Abhandl. der k. Akad. der Wissensch. zu Berlin, 1814-1815. 

 s. 18-26. Naumann Lehrbuch der Geognosie Band 1, 2d ed. 1858, s. 597. 



