56 VOLCANIC ROCKS OF SOUTH MOUNTAIN. [bull. 136. 
rocks are pierced with coarse quartz veins which, bear native copper. 
Copper in microscopic quantities and copper oxide also occur m the 
amygdules. Hand specimens are frequently coated with the copper 
carbonates, malachite and azurite. There were picked up on the road- 
side some specimens of amygdaloidal aporhyolites that are quite 
diverse from the amygdaloids of the Bigham copper mine which have 
just been described. They are similar to specimens found north of the 
Monterey district at Eaccoon Greek. The amygdules, which are black 
against a yellowish- white background, are finely attenuated and elon- 
gated in long parallel hair lines, lending to the rock the appearance of 
an eutaxite. The black color is due to magnetite, which either is finely 
disseminated in quartz (the other infiltrated mineral), or is present in 
masses, or simply forms a heavy rim around the amygdules. 
Some of the amygdaloids from Eaccoon Creek merit a detailed descrip- 
tion. In these rocks the vesicles are usually bordered by a broad rim 
like the groundmass, in its present crystallization, but separated fruin 
it by a narrow, clear zone of quartz, and characterized by a greater 
abundance of magnetite (or ilmenite). On the inner edge of this 
border are spherulitic growths, while the rest of the vesicle is filled 
with quartz (PI. XXVII, b) or with quartz and an opaque black oxide 
(PI. XXVII, a). In the latter case the black oxide occupies the center 
of the vesicle, leaving a clear zone of silica around the sxmerulites. 
Crossed nicols show that the spherulites are optically continuous with 
the quartz, and that the radial appearance which has been retained is 
due to the arrangement of the impurities. The appearance of these 
vesicles is very suggestive of those figured by Professor Cole. 1 Pro- 
fessor Cole explains this type of spherulite by a dual mode of growth — 
a radial growth outward from the groundmass, as well as inward, origi- 
nating in the glass and converging toward a spherulitic center ! He 
does not give the mineral character of the spherulite. Whatever may 
be the facts with reference to the Eocche Eosse obsidian, it is not 
necessary to postulate an abnormal method of crystallization to explain 
the phenomena observed in the South Mountain aporhyolites. 
The spherulites projecting into the vesicle, with their bases sunk into 
its walls, were recognized by Professor Iddings, who kindly examined 
the section, as tridymite spherulites, such as form on the walls of vesic- 
ular cavities in all kinds of modern lavas. Tangential sections of such 
spherulites are represented by granular aggregations. The form of 
the tridymite has been preserved by impurities, while its molecular 
arrangement has been altered to that of quartz. The presence of a 
border between the groundmass and the cavity suggests that crystal- 
lization, starting from the walls of the cavity, took place within the 
magma, initated, perhaps, by the gaseous content of the vesicle. 
'Grenville A. J. Cole and Gerard W.Butler, On the lithophyses in the obsidian of the liocche 
Rosse, Lipari : Quart. Jour. Geol. Soc. London, Vol. XLVIII, 1892, p. 438. 
