42 THE ROMAN COMAGMATIC REGION. 
rare. Their color is inclined to yellowish, the luster is apt to be somewhat waxy, 
and they are mostly quite free from inclusions. There are also very few large (10 to 
20 mm. long), stout prismoids of feldspar, which the microscope proves to be of 
orthoclase, and some smaller ones of what the thin section shows to be labradorite. 
In addition are very few and small prismoids of black augite, so inconspicuous as 
almost to escape notice. The groundmass is a rather dark ash gray, and quite 
aphanitic. 
Microscopic characters. In thin sections the larger leucite phenocrysts have 
generally fallen out, but the fragments left show the usual double refraction and 
twinned structure. One or two sections of the large feldspar phenocrysts show that 
they are of orthoclase, undoubtedly sodic, while the smaller ones are of labradorite, 
about Ab x An 2 . These last are much twinned, and are usually gathered into clusters, 
whose numbers are greater than those of the orthoclase phenocrysts. The pheno- 
crysts of augite are subhedral or anhedral, mostly in stout prismoids or fragments, 
and of the usual pale-gray tint. 
The groundmass is holocrystalline, and the fabric is, on the whole, trachy- 
tic, though not as typically so as in many other types, on account of the presence of 
considerable numbers of formless anhedra of orthoclase and rounded leucites. It 
is composed very largely of feldspar, for the most part in slender prisms or laths, 
of which the majority are of orthoclase with fewer of labradorite, though it was 
impossible to arrive at any satisfactorily accurate estimate of the relative amounts. 
As noted above, anhedral individuals of orthoclase also occur. Leucite is present 
in considerable amount as small, round anhedra, denned by the rings of augite 
and magnetite grains and feldspar laths surrounding them, and identifiable by the 
faint double refraction. Scattered through the mass are the usual small prismatic 
anhedra of colorless augite, equant anhedra of magnetite, and rare apatite prisms. 
Neither nephelite nor glass could be detected, and the sodalite minerals seem to be 
wholly absent. 
Chemical composition. For the analysis a fair-sized hand specimen was crushed 
and sampled, in order to obtain a representative powder, on account of the large 
size of the leucite phenocrysts. With the analysis of mine is given one by Ric- 
ciardi of a rock also from Proceno, called a leucite-tephrite by Klein, whose descrip- 
tion agrees in the main with that of the specimen analyzed by me. There is also 
given an analysis by Riva of a bolsenal vulsinose-pulaskose (vulsinite) from the 
Astroni Volcano in the Phlegrean Fields. 
The analysis itself does not call for special remark, except that the consider- 
able amount of BaO and the presence of small quantities of zirconia, strontia, and 
the ''rare earths" may be noted. These last were determined with care by Hille- 
brand's method,* and there is probably little doubt that they exist in similarly small 
amounts in other rocks of the region, as they were also found to be present in a more 
femic rock. 
* W. F. Hillebrand, Bull. 176, U. S. Geol. Surv., p. 77, 1900. 
