36 VOLCANIC ROCKS OF SOUTH MOUNTAIN. [bull. 13(1 
South Mountain and the typical quartz -pophyries is not one of the 
absence or presence of phenocrysts. Thus the term felsite would be 
forced to cover not only nonporphyritic acid rocks, but also conspicu- 
ously porphyritic rocks. Finally, felsite, though useful as a field name, 
may well be objected to as an inaccurate petrographical term. 
A brief history of the word felsite and its synonyms in different coun- 
tries (petrosilex, eurite, and halleninta,) will serve to illustrate the 
unfitness of it and these allied terms for exact petrographical usage. 
"Felsit" was introduced into German petrographical nomenclature 
by Gerhard 1 in 1814, who applied the term to a matrix which he claimed 
was common to all feldspar, claystone, and hornstone porphyries. This 
matrix was a compact homogeneous aggregate of feldspar and quartz, 
supposed by Gerhard to be compact feldspar. 
In 1858 Naumann' 2 adopted Gerhard's word for the feldspar-quartz 
groundmass, and called all porphyries with such a base felsite-porphy- 
ries. This term has since been applied by Tschermak to porphyries 
without quartz phenocrysts (orthofelsites, orthophyres). 
The more accurate German usage of the present day discards felsite 
save as a macroscopic term for an unresolvable porphyry base, 3 while 
"felsitfels" is used if, in the absence of phenocrysts, the rock is com- 
posed of this base only. 4 
This same quartz-feldspar mosaic, confused with compact feldspar, 
was distinguished as petrosilex by Wallerius 5 as early as 1747, and 
subsequently by Dolomieu. This term was also early used by Bron- 
gniart. In 1819 the same groundmass was called eurite by Daubisson * 
because of its fusibility. Michel-Levy 7 uses the term petrosilex, but 
with a more limited meaning. He defines it as essentially synonymous 
with Eosenbusch's microfelsite, making "porphyre petrosiliceux" equiv- 
alent to felsophyre. Petrosilex is, however, generally used by Levy and 
other French petrographers in a more or less loose and vague way, to 
cover a crystalline or cryptocrystalline quartz -feldspar aggregate or a 
partially amorphous siliceous feldspathic magma. 
In Sweden, the fine-grained, apparently homogeneous acid rocks 
consisting essentially of quartz and feldspar and rarely porphyritic, 
are called halleninta. 8 They may be of aqueous or of igneous origin. 
The early meaning of felsite in England was quite similar to that 
given it on the Continent. According to Pinkerton 9 the term was 
'Beitriige zur Geschichte des Weissteins ties Felsit und anderer verwaiidten Arten: Abhandl. 
K. Akad. Wiss. zu Berlin, 1814, 1815, pp. 18-2G. 
2 Lebrbuch der Geognosie, 2d ed., Vol. I, 1858, p. 597. 
3 Rosenbuseh, Mikro. Pliys., etc., 2d ed., Vol. II, p. 373. 
4 Rosenbusch, loc. cit., p. 351. 
5 Systematica Mineralogicum, 1847, French translation, 2 vols., Paris, 1853. 
G Traite do geognosie, lsted., Vol. I, 1819, p. 112. 
7 Structures et classification des roches eruptives, p. 17. 
8 Justus Roth, Allgeraeine und chemiscbe Geologie, Vol. II, p. 494. F. Zirkel, lyehrbucb der Pet> 
rographie, Vol. I, p. 564. 
9 T. Pinkerton, Petralogy, A Treatise on Rocks, Vol, I, 1811, p. 161. 
