74 THE ROMAN COMAGMATIC REGION. 
II. 5. 2. 3. Arsal Monzonose [Vulsinite, Arso Type]. 
Megascopic characters. Rocks of this type are very similar to those belonging 
to arsal vulsinose and ciminose described above, both megascopically and micro- 
scopically; so much so, indeed, that they are quite indistinguishable in the hand 
specimen. 
The color is a pure gray which may vary from rather light to rather dark. Glis- 
tening, white, prismatic phenocrysts of feldspar, from 5 to 20 mm. long, are rather 
abundant and are more or less conspicuous according as the groundmass is dark 
or light. With these are seen, but in very small amount, small black prisms of 
augite, and in still less quantity either small tables of biotite or grains of yellow 
olivine. The groundmass is dense and aphanitic. Most of the rocks of this type 
in the Roman Region are compact, but occasionally more or less vesicular forms are 
found. 
Microscopic characters. The feldspar phenocrysts are seen in thin section to 
be of both alkali and soda-h'me feldspar, the amount of the former surpassing that 
of the latter. While the orthoclase is more or less sodic it does not show any of 
the microperthitic or other micro-structures which are so common in soda-ortho- 
clase. The stout prisms are euhedral or subhedral, often twinned according to the 
Carlsbad law, and carry few inclusions. Those of soda-lime feldspar are similar in 
form and show the usual multiple twinning. The extinction angles vary somewhat, 
but in most cases indicate that the composition is near the border of andesine and 
labradorite, varying from Ab 4 An 3 in the Arso rock to Ab 3 An 4 in that from Poggio 
Cavaliere, and even slightly more calcic. It will be recalled that in the homologous 
types of vulsinose and ciminose the plagioclase was a labradorite about Ab z An 2 , or 
even an almost pure anorthite, the composition being, of course, correlated with 
the dopotassic or sodipotassic character of the subrang. 
The augite phenocrysts call for little remark. They vary in length from i to 
5 mm., are usually anhedral, in stout prismoids or fragments of these, and are of 
the common very pale-gray color, sometimes slightly greenish-yellow. The rare 
biotite phenocrysts, which are not present in all specimens, are in thick tables, i to 
3 mm. wide, of the common brown color and usually much altered. In some 
specimens, as that from L'Arso, these are replaced by phenocrysts of olivine, from 
i to 2 mm. in diameter, mostly anhedral, occasionally with crystal planes, colorless 
in thin section, and quite fresh. 
The groundmass is composed very largely of slender laths of feldspar, those of 
orthoclase preponderating over those of soda-lime feldspar, which is an andesine- 
labradorite. In the Arso rock these small laths are branched at the ends and often 
slightly curved. Their arrangement is subparallel, giving rise to a trachytic fabric. 
With them are seen rarely, as in the Arso specimens, very small, round anhedra of 
leucite, but the amount of this is quite negligible, not over 2 per cent, and it is note- 
worthy that it does not occur in those rocks which show biotite phenocrysts, as that 
from Poggio Cavaliere. The presence of leucite in the Arso rock was first noted by 
