bascom.] APORHYOLITES. 55 
Irving.' A perlite from Deer Creek Meadows, 25 miles south of Lassen 
Peak, displays a similar rliyolitic structure. This structure is essen- 
tially a phase of the fluidal structure. 2 
Lithophysal structure. — Very often the macroscopic features of the 
aporhyolites disclose their original character more convincingly than do 
the microscopic. Lithophysal are best revealed in the hand specimen? 
where they are brought out in delicate relief by weathering. In such 
specimens from the Raccoon Creek locality, the rose-pink petals of the 
lithophysa' in a pale pink base produce quite as beautiful examples of 
this glassy structure as any obsidian or rhyolite offers. (PL XI.) No 
undoubted lithophysal were found within the Monterey district. The 
microscope discloses some vesicular structures which bear slight trace 
of a lithophysal character, but the alteration has been too great to 
allow of their identification as hollow spherulites. 
Mieropcgmatitic structure. — Themicropeginatitic structure shows itself 
in microscopic pegmatoid groups of phenocrysts, such as have been 
frecpiently described in porphyries and rhyolites. 5 It does not play an 
important part in the aporhyolites. 
Perlitic structure. — That this structure is present in the South Moun- 
tain rocks, and in great perfection, has already been noted. (PI. XX, a, 
PI. XXI, a.) While its presence is a most reliable proof of the former 
character of the rock, its absence furnishes no evidence against the pre- 
vious glassy condition of the rock, both because many recent rhyolites 
showed no trace of that structure and because it is most readily effaced 
by devitrification. 
Amygdaloidal structure. — At Paccoon Creek, at the Bigham copper 
mine and its near vicinity, are light-colored (pink and yellow), extremely 
vesicular aporhyolites. The vesicles are oval or elongated by How move- 
ment. (PI. XXVI, a and b.) They are uniformly filled with epidote or 
quartz, with both, or with either to the exclusion of the other. When 
both are present the quartz forms a rim around the epidote. The epidote 
has often a radial arrangement, while crystal boundaries are absent. Its 
color varies from a deep yellowish green to light yellow, and pleochro- 
ism is marked. In some of the larger amygdules the radiating needles 
of epidote have been broken and pulled apart at right angles to their 
longer axis and the spaces filled with silica. Piedmontite and quartz 
show the same relation, as described on page 41. The groundwork of 
these amygdaloids is the usual holocrystalline quartz-feldspar aggre- 
gate. Incipient alteration to granular epidote is more frequent in these 
open- textured amygdaloids than in the compact aporhyolites. These 
■Irving, Copper-bearing rocks, etc.: Mon. U. S. Geol. Survey, Vol. V, pp. 312-313, fig. 22. 
2 Thisisthe " Aschenstructur " of Miigge (Untersuehungen fiber dio "Lenneporphyro'' in Westfalen 
und den angrenzenden Gebieten: Neues Jalirb. fur Min., Geol. u. Pal., B. B. VIII, 1S93, pp. 048, G19, 
713), who considers ifc due to the original fragmental character of the lava. Whether it is always to 
be so interpreted is a question for further investigation. 
1 Iddings, op. cit., p. 275, PI. XV, fig. 5. 
