PETROGRAPHY. 27 
CUMAL PHLEQROSE. I. 5. 1. 3. 
Megascopic characters. Light gray, compact, slightly porphyritic. Feldspar phenocrysts 
rare, i to 5 mm., stout prismatic or tabular, clear and colorless. Sodalite or haiiyne pheno- 
crysts rarer, not always visible, about i mm., colorless or blue, equant. Augite phenocrysts 
very rare, about i mm., prismatic, black. Groundmass: aphanitic, light gray, usually dull, 
but sometimes with slightly greasy or silky luster, often mottled with darker schlieren. 
Microscopic characters. Holocrystalline, megaporphyritic, perpatic, mediophyric. Phen- 
ocrysts: 10 per cent or less, soda-orthoclase, sodalite or haiiyne, aegirite-augite. Groundmass: 
90 per cent or more, soda-orthoclase, sodalite or haiiyne, nephelite, aegirite-augite, magnetite, 
titanite, occasionally a little glass. 
Soda-orthoclase, Or 2 Ab t to Or t Ab,. Phenocrysts: about 5 percent, i to 5 mm., euhe- 
ral to subhedral, tabular parallel to b (oio) or stout prismatic, microperthite and microcline 
structures wanting, Carlsbad twinning common, inclusions rare. Groundmass: about 90 per 
cent, 0.05 to 0.20 mm., subhedral prismatic, arrangement parallel or subparallel. 
Sodalite or haiiyne. Phenocrysts: about 3 per cent, 0.5 to 2.0 mm., subhedral, equant, 
colorless or blue, inclusions sometimes common, dusty, in fine lines. Groundmass: about 2 
per cent, same as phenocrysts but much smaller. 
jEgirite-augile. Phenocrysts: about 2 per cent, 0.5 to 2.0 mm., subhedral, prismatic, 
pale olive-green, pleochroic. Groundmass: about 2 per cent, 0.05 to o. 10 mm., subhedral, pris- 
matic, pale green or gray. 
Magnetite. Groundmass: about 3 per cent, 0.05 to 0.5 mm., anhedral, equant. 
Titanite. Groundmass: about 0.5 per cent, 0.5 mm., euhedral. 
Nephelite. Groundmass: about 5 per cent, formless areas, interstitial between the feld- 
spar prisms, colorless. 
Chemical composition as in analyses I, II, and III, p. 23. 
Type specimens from Monte di Cuma and Monte Nuovo, Phlegrean Fields, and Monte 
Vico, Ciminian District. 
I. 5. 1. 3. Rotaral Phlegrose [Trachyte Obsidian, Rotaro Type]. 
Megascopic characters. Megascopically these rocks show rather numerous 
(about 15 per cent) white phenocrysts of alkali-feldspar, in stout prisms, from 
i to 5 mm. long, and which stand out prominently against the dark groundmass. 
The very dark gray or black groundmass is either obviously hyaline to the naked eye 
or else aphanitic and with a rough fracture; but even in this last case examination 
with the lens and the natural surfaces of the lava blocks shows that it is highly vitreous. 
Microscopic characters. In thin section the feldspar phenocrysts are clearly 
of a sodic orthoclase, usually with Carlsbad twinning, and the few small phenocrysts 
of augite show a very pale gray or yellowish green and those of biotite a brown 
color. The groundmass varies considerably, enough so as to permit the establish- 
ing of several distinct types, were this desirable. In some cases, as in specimens 
from the Monte di Procida, as well as some Ischian localities, it is an almost pure 
glass of a light-brown color and quite free from microphenocrysts or microlites. 
In others, as in the Monte Rotaro rocks most typically, it is quite thickly strewn 
with small, prismatic microphenocrysts of sodic feldspar, which are sometimes much 
branched. It was noticed that the rocks in which the glass cement is most free 
from these small crystals are very friable and break into small pieces under the 
hammer, not showing the solidity and coherence of such well-known obsidians as 
those of Lipari, Iceland, or the Yellowstone Park. 
