112 THE ROMAN COMAGMATIC REGION. 
transitional toward the clathrate one are not uncommon, so that it is, at times, diffi- 
cult to say to which type some rocks belong. 
The leucite, which constitutes about 40 per cent of the rock, is in subhedral to 
anhedral individuals, sometimes rounded and again rather irregular, interstitial 
between the augites. It is, however, usually equant, and often contains small 
inclusions of augite, magnetite, or glass, arranged zonally. Skeletal forms are rarely 
seen. In size the leucites vary much, though they are always microscopic, from 
0.05 to 0.30 mm., some of the more irregular patches being larger than this last. 
The augite, of the usual pale-gray color, is almost as abundant as the leucite 
and is phenocrystic to some extent, in subhedral prismoids, often "corroded" and 
with irregular outlines, of from 0.5 to i mm. in length. But for the most part it 
forms subhedral to quite anhedral individuals, either stout prismoids or irregular 
grains and interstitial patches between the leucites. These augites are of dimen- 
sions commensurate with the leucites, from o.i to 0.5 mm. long, and they are much 
stouter, larger, and more equantly developed than the small augites of the galeral 
type, in which the augite form is typically slender prismatic. Furthermore, they 
lack the felted arrangement and tangential position toward the leucites of this last 
type. With the above minerals are small grains of olivine and magnetite and very 
small apatite needles. 
Lying between the larger leucites and augites, and inclosing the smaller crystals, 
is a colorless cement which the use of crossed nicols resolves into three distinct sub- 
stances. Part of it is isotropic, of a slightly lower refractive index than the leucite, 
and may be considered a glass. Again, it is feebly doubly refracting without 
twinning lamellae, probably nephelite, the presence of which is indicated by the 
calculation of the mode from the norm. Elsewhere this base consists of anhedral 
individuals of feldspar, the irregular patches of which extinguish simultaneously 
over considerable areas, and which inclose poikilitically the smaller leucites and 
augites. A small proportion of this feldspar is orthoclase, distinguished by its 
refractive index and by the simple Carlsbad twinning; but most of it is a multiply- 
twinned labradorite, of about the composition Ab z An 3 . 
The origin of these poikilitic feldspars is somewhat problematical. In most 
of my specimens, especially those from Arcioni (one of which was chosen for analy- 
sis), I can not but regard it as primary, that is, one of the last products of crystalliza- 
tion, but antecedent to the nephelite and glass base. On the other hand, Sabatini* 
and Viola,f who have studied it, the former in the Latian lavas and the latter in those 
of the Hernican District, regard it as secondary. This conclusion is based on the 
observations made by them that many individuals with the characteristic forms and 
inclusions of leucite show, between crossed nicols, the birefringence and multiple twin- 
ning of plagioclase, which varies in different cases from an oligoclase to anorthite. 
The twinning lamellae pass out uninterruptedly from the apparent leucite crystal 
into a surrounding colorless patch of feldspar, of identical optical properties, extin- 
* V. Sabatini, Boll. Soc. Geol. ltd., X (1896), p. 70, also Mem. descr. Carlo, Geol. TtaJ., X, 1900, pp. 155, 263, 276. 
t C. Viola, Boll. Com. Geol. Ital., 1896, pp. 14, 23; Proc. Soc. Tosc., 1896, p. i; Neu. Jahrb. 1899, I, pp. 127, 131. 
