60 VOLCANIC ROCKS OF SOUTH MOUNTAIN. Lbitll.136. 
which, like the South Mountain volcanies, were once described as sedi- 
mentary, are proving to be acid volcanies, preserving the features of 
their modern equivalents. Quite recently glassy and rhyohtic struc- 
tures in these rocks Lave been observed and described by Otto Nor- 
denskjold. 1 
In Belgium, de la Vallee-Poussin seems to be the only writer who has 
brought out the resemblance between the eurites of that country and 
modern rhyolites. He describes at some length structures similar to 
those possessed by the aporhyolites of South Mountain. A vacillating 
state of mind as to the matter of nomenclature is indicated in the 
titles of his successive papers. 2 
In England the rhyolitic character of ancient acid volcanies has been 
recognized and emphasized, and the idea of devitrification is widely 
accepted. Allport, Cole, Bonney, Butley, Judd, and Harker have 
accomplished most valuable work along this line. 
Dr. Wadsworth 3 was the first American petrographer to advocate 
the abandonment of age as a factor in rock classification, while at the 
same time he recognized devitrification as the process which was form- 
ing felsites out of rhyolites. What he says is of interest in its anticipa- 
tion of ideas now more generally accepted: 
This devitrification gives rise in the older and more altered rhyolites to the feld- 
spar, quartz, and microfelsitic (so called) hase that has so puzzled lithologists in the 
study of the felsites. The rhyolites of all volcanic rocks preeminently show lamina- 
tion produced by flowing, a fact which is doubtless duo to their being so siliceous. 
This structure and their devitrification enable us to trace a direct connection 
between the rhyolites and felsites, which are simply the older and more altered rhy- 
olites. * * * Q ne f the best illustrations of this is to be found on Marblehead 
Neck, Massachusetts, where at least two distinct flows of felsite occur, one cutting 
the other. They show the fluidal structure so characteristic of rhyolites — a char- 
acter that has been mistaken for lines of sedimentation by geologists, while the 
inclosed crystals of orthoclase have been taken for pebbles. While to the 
naked eye and under the microscope this rock shows the fluidal structure of a rhy- 
olite, iu polarized light it i3 seen that the base has been completely devitrified, a 
process that is carried to a great extent in many known modern rhyolites. 
No other American petrographer has so distinctly advocated the 
identity of felsites and ancient rhyolites, in spite of the fact that many 
of our felsites illustrate it as unmistakably as do the English felsites. 
Dr. Irving, 4 in his description of the Beaver Bay group of the Kewee- 
naw series, repeatedly calls attention to the resemblance between the 
ancient felsites and quartz-porphyries and the modern rhyolites, though 
he does not express an opinion as to their equivalence. 
1 Op. cit. Also Ueber arclueische Ergussgesteine aus Smaland; Bull. Geol. Inst Upsala, No. 2, 
Vol I, 1893, pp. 1-127. 
; Les ancienncs rhyolites, elites eurites, tin Grand-Manil : Bull. Acad. roy. Belgique, 3d series Vol. X, 
1885, pp. 253-315. Les eurites quartzeuses (rhyolites anciennes) de Nivelles et des environs: Bull. 
Acad. roy. des sci. et des beaux-arts de Belgique, 57 aunee, 3d series, Vol. XIII, No. 5. 1887. 
;/ M. E. Wadsworth, Notes on the mineralogy and petrography of Boston and vicinity: Proc Boston 
Soc. Nat. Hist., Vol. XIX (May, 1877), p. 236. On the classification of rocks : Bull. Mass. Comp. Zool. 
Harvard Coll., Vol. V, No. 13, June, 1879, p. 277. 
4 0p. cit., pp. 312, 313, note 5, p. 436. 
