bascom.] APORHYOLITES. 57 
Somewhat similar radial growths within vesicles in ancient rhyolites 
have been described and figured by de la Vallee-Poussin. 1 
Taxitic structure. — Another structure which the South Mountain rocks 
possess in common with rhyolites is what has been called the taxitic. 2 
This consists in the intimate mingling of two portions of the magma 
which from some cause (liquation) are slightly differentiated. The 
iron constituent, which evidently separated out in the original glass, 
has been still further crowded into bands and curved lines by the 
secondary crystallization. The result is the production, in some cases, 
of an irregular mottling, when the rock is called an ataxite; and in 
other cases of a more or less complex network of interlacing bands 
following lines of flow and forming a eutaxite. This mottling and 
banding is made the more striking by a marked contrast in color. The 
body of the rock is light-gray or pink, and the lines are dark blue-gray 
or red, according to the varying degrees of oxidation of the iron. 
Where the dark lines outline oval and spherical spaces and contain 
porphyritical crystals iu or near their centers, the crystallization is 
regarded as having once been spherulitic and the rock is termed a 
spherutaxite. These have been described on page 50. 
The eutaxites are frequently so sheared as to give a hair-like tenuity 
to the bands in cross section, while the microscopic slide shows the 
effect of pressure on the rock in the parallel arrangement of the glob- 
ulites of black oxide. The universal presence of globulites, trichites, 
and microlites of black and red iron oxide in flow bands, or indiffer- 
ently distributed, or in concentric zones around spherulites and vesi- 
cles, is worthy of mention as a further point of resemblance to the mod- 
ern rhyolite. Such a trichitic structure in similar rocks has been 
described by various petrographers. 3 
SUMMARY OF PROOF OF DEVITRIFICATION. 
It is not easy to present the evidence for the secondary nature 
of the holocrystalline groundmass so that it shall have the weight 
which properly belongs to it. Very much depends upon effects which 
it is impossible to convey by description, but which carry conviction 
to the student of these rocks. The contrasting appearance of many of 
the sections in ordinary and polarized light can not be adequately repro- 
duced. The disappearance under crossed nicols of rhyolitic, perlitic, 
'Lesanciennes rhyolites, elites eurites, de Grand-Mauil : Bull. Acad. roy. Belgique, Lid series, Vol. X, 
1885, p. 292. 
"Fritsch and Reisa, Teneriffe, 1808, p. 414. 
Roaenbusch, Mic. Phys. der Massigen Gesteine, 2d ed., p. 625. 
F. Loewinson-Lessing, Zur Bildungsweise uiid Classification der klastischen Gesteine, 1888, pp. 228- 
235. Note sur les taxitea et aur lea roches clastique vulcanici tie : Bull. Soe. Belg. geol., etc., Vol. V., 
1891 Loewinaon-Lessing's division of the taxitea into ataxites, eutaxites, and spherutaxitea lias 
been followed in this bulletin. 
S R. D. Irving, Copper bearing Rocks, etc. : Mon. U. S. Geol. Survey, Vol. V, p. 312. 
S. Allnort, On certain ancient devitrified pitchstones and perlites from the Lower Silurian district 
of Shropshire. Quart. Jour. Geol. Soe. London, Vol. XXXIII, p. 449. 
Nordenskjold, op. eit. 
