bascom.] APORHYOLITES. 49 
cate arborescent clusters above mentioned. Whether this secondary quartz may ever 
be rather a result of devitrification than a truly secondary or alteration product, I 
have no means of deciding, though it is certainly the latter often, and I should sup- 
pose always. It surely can have no connection with the original solidification of the 
rock. 
Observations made on the South Mountain aporhyolites lead to 
essentially the same conclusions as those reached by Irving. As the 
nature of the structure is of both interest and importance in its bear- 
ing upon the question of the primary or secondary character of the 
groundmass, attention will be called to some of the observations which 
prove suggestive. 
It has been stated that a few sections of the basic lavas of South 
Mountain exhibited this structure. In these sections the nature of the 
structure could be more readily detected. (PI. XIX, a and b.) 
The outline of the lath-shaped feldspars, forming an original ophitic 
structure, is completely preserved, though none of the original constit- 
uents of the rock remain, unless some of the titaniferous iron oxide is 
original. 
At present the rock consists entirely of cpiartz, epidote, magnetite 
(or ilinenite), aud leucoxene. It is amygdaloidal, and the vesicles are 
filled with secondary quartz. Quartz is also a cement for the minerals 
of the groundmass, and forms irregular interlocking areas, which are 
quite similar to the micropoikihtic areas of the finer-grained acid rocks, 
and produce in polarized light the familiar patchy effect. Fine cracks 
traversing the sections are still preserved in outline by the ferrite, but 
are prior to the quartz areas, which have obliterated all trace of cement- 
ing material. The epidote, which replaces crystals of some former ferro- 
magnesian constituent, is often pierced by these cracks, which become 
invisible in the quartz areas save for the outlining ferrite. There can 
be no question as to the secondary character of the micropoikihtic 
structure in these cases. 
Some structures described by Zirkel l in the rhyolites of the Washoe 
district are suggestive in this connection. He notes (slide 350, fig. 1, 
PI. VI) perlitic parting in certain rhyolites (southeast from Wadsworth), 
where the cracks, semicircular and oval, traverse a glassy groundmass 
and are bordered by a narrow zone of microfelsite, "giving [to the sec- 
tion] the appearance of a network." The same general effect is produced 
in other rhyolites of the district (sees. 351, 352, figs. 2, 3, PL VI) by 
faint granular lines ''which, by their fluidal running, form a net with 
a multitude of meshes of oval shape." These lines seemed to be the 
vestiges of perlitic cracks, though they could not be certainly deter- 
mined as such. A widespread and characteristic type of rhyolite (sees. 
333, 407, fig. 1, PI. VIII) shows the same network of dark granular lines, 
but in this case the meshes are filled with a spherulitic crystallization. 
There are, then, two types of crystallization which may occupy the 
1 Geol. Expl., 40th parallel, Vol. VI, Microscopic Petrography. 
Bull. 13G 4 
