bascom.] NOMENCLATURE. 37 
introduced in 1794 by Kirwan l for compact feldspar. Current usage as 
defined by Teall 2 applies the term to u compact stony rocks, the min- 
eral composition of which can not be ascertained by examination with 
the naked eye or with the lens. * * * These rocks are anhydrous 
(or nearly so), and except in this respect agree in composition with the 
acid glassy lavas." 
Dana 3 (as an exponent of American usage) defines "felsyte," a rock, 
as " compact orthoclase with often some quartz intimately mixed, fine 
granular to flint-like in fracture, * * * both metamorphic and 
eruptive." The mineral he defines as follows : " Felsite is compact, 
uncleavable orthoclase, having the texture of jasper or flint, which it 
much resembles. It often contains some disseminated silica. It is 
distinguished from flint or jasper by its fusibility. * * * It is the 
base of much red porphyry." This is substantially the " felsitfels " and 
r felsit" of Eosenbusch. 
In short, " felsite" has been used to describe an acid base, unresolvable 
by the naked eye, and once supposed to be a single mineral. With the 
Introduction of the microscope this macro" felsitic " base was resolved 
into the microgranitic, micropegmatitic, and microfelsitic groundmass, 
the point of ignorance having been shifted from the felsitic base, 
macroscopically unresolvable, to the microfelsitic base, which is micro- 
scopically unresolvable. 4 
On the Continent " felsite" has practically been replaced by these 
terms. British and American petrographers retain it as a useful field 
name for rocks formed of this macroscopically unresolvable base with- 
out phenocrysts or with inconspicuous phenocrysts. In this sense the 
word will be used, when used at all, in this paper. 
It is very generally recognized that structural features are not con- 
ditioned by the geological age of rocks, but are a function of the condi- 
tions of consolidation. That these conditions, while very varied and 
complex in any geological period, have not essentially altered since 
Paleozoic times, has been shown to be the case by some of the most 
able observers. 5 
With this recognition has come the growing conviction among petro- 
graphers that mere age should be eliminated as a factor in geolog- 
ical nomenclature. While this is true, it is felt on the other hand 
that the rock name should show some recognition of the alteration 
'Richard Kirwan, Elements of Mineralogy. 
S J. J. Harris Teall, British Petrography, p. 291. 
:< J. D.Dana, Manual of Mineralogy and Lithology, 3d ed.. 1883, pp. 280, 442. Manual of Geology, 
3ded., 1879, pp.73, 77. 
"Rosenbusch does not assent to this interpretation of microfelsite, hut regards it as a definito 
chemical compound allied to feldspar, just as felsite was once regarded. 
! > Teall, Address of the Pres. of the Geol. Soc, of the British Ass. Adv. Sci., 1893. 
Judd, On the gahhros, dolerites, and basalts of Tertiary age in Scotland and Ireland: Quart. Jour. 
Geol. Soc., London, Vol. XLII, 1886, pp. 49-97. 
Allport, Tertiary and Paleozoic trap rocks: Geol. Mag., London, 1873, p. 196. British Carboniferous 
Dolerites: Quart, Jour. Geol. Soc, London, Vol. XXX, 1874, pp. 529-567. 
Iddings and Hague, On the development of crystali/.ation in the igneous rocks of Washoe, Nev., 
with notes on the geology of the district . Bull. U. S. Geol. Survey No. 17, 1885 
