PETROGRAPHY. 29 
The very close resemblance between this analysis and the preceding ones is 
evident, though the norm shows that this rock is nearer all the center points of 
phlegrose than any of the others. This is especially true of the ratio of soda to 
potash, which is very close to unity, instead of approaching the dosodic border as 
in the other phlegroses of the district. 
As the mode is indeterminate through the presence of abundant glass, it is not 
possible to discuss the relations of norm and mode. It may be observed, however, 
that the greater part of the normative augite and magnetite molecules, with the 
sodalite, have not crystallized out, but exist in the glass base. Were the rock holo- 
crystalline, its mode would have been undoubtedly almost identical with that of the 
Cuma phlegrose, and the average feldspars would have had the composition 
Or 1 Ab I . 
Occurrence. In the Roman Region the rotaral type is found only in the Cam- 
panian District, and even here is by no means abundant. Among localities which 
may be mentioned are Monte di Cuma and Monte di Procida in the Phlegrean 
Fields, and Monte Rotaro and Scanella on Ischia. It is noteworthy that this highly 
vitreous texture is never assumed by the same magma in the other districts where 
it occurs, the Auruncan and the Ciminian, where the phlegrose rocks are uniformly 
holocrystalline. 
Name. The type name is derived from Monte Rotaro, the locality of the type 
specimen. In prevailing classifications this type would, of course, be considered a 
trachyte-obsidian. 
ROTARAL PHLEQROSE. I. 5. 1. 3. 
Megascopic characters. Compact, black sprinkled with white crystals, obviously highly 
vitreous, porphyritic. Phenocrysts of feldspar rather abundant, i to 5 mm., stout prisms, white 
and colorless, prominent. Groundmass black or brownish black, highly vitreous, often with 
conchoidal fracture, sometimes rough. 
Specific gravity, 2.44. 
Microscopic characters. Dohyaline, mega- and micro-porphyritic, dopatic, mediophyric. 
Phenocrysts, about 15 per cent, feldspar. Groundmass: about 85 per cent, mostly glass, with 
some microphenocrysts of alkali -feldspar, augite and biotite. 
Soda-orthoclase. Megaphenocrysts: about 15 percent; i to 5 mm.; subhedral to euhedral, 
stout prismatic; Carlsbad twinning rare, inclusions rare. Microphenocrysts very variable, 
sometimes about 50 per cent, sometimes almost none; o. 10 to 0.30 mm. ; subhedral, prismatic, 
often branched in sheaflike forms; arrangement often subparallel. 
Augite. Microphenocrysts: about 2 per cent; o.i to 0.2 mm.; euhedral, prismatic; 
pale gray or very pale greenish. 
Biotite. Microphenocrysts: about i per cent, not always present; 0.05 to 0.02 mm.; thick 
tabular; brown, unaltered. 
Glass. About 80 per cent; usually brown, sometimes almost colorless. Sometimes dusty 
with minute, indeterminate microlites. 
Chemical composition and norm as on p. 28. 
Type specimen from Monte Rotaro, Ischia. 
