132 THE ROMAN COMAGMATIC REGION. 
i. n. 
Anorthite 5.3 none 
Leucite 30.0 16 
Nephelite 5.8 none 
Analcite none 4 
Zeolites none 4 
Augite 50. 4 50 
Olivine 2.3 15 
Melilite 3.4 none 
Biotite trace 6 
Ores , 1.8 5 
Apatite i . o .... 
100. O 100 
The figures of I correspond with the appearance in thin section, and may be 
regarded as approximately correct. As compared wth the missoural type the amount 
of augite in each is the same, and the nephelite of I is replaced by analcite and zeolites 
in II. The chief differences are found in the much higher leucite and lower olivine 
of I, as well as in the presence in this of anorthite and melilite and in II of biotite, 
all in small amounts. It is clear that the mode of I is practically normative, the re- 
adjustments needed to form augite and melilite being negligible, while the mode of II 
is more abnormative, about one-quarter of the normative leucite going into biotite.* 
The romal type may then be described as normative salphyri-albanose, while the 
missoural type would be a biotitic grano-albanose. 
Occurrence. This type is abundant in certain districts, especially in the Vul- 
sinian, the Sabatinian, and the Latian. In the first some localities are near San 
Lorenzo and at the cemetery of Bolsena, at the upper flow in the quarry on the west 
side of Monte Rado, and at the Ponte della Regina and Madonna del Borgale, south- 
east of Lake Bolsena. In the Sabatinian District the most prominent locality is 
the crater of Trevignano, on the north shore of Lake Bracciano. Most of the exten- 
sive flows south of the lake with a similar habit belong to braccianose, as we have 
seen, but at Casaccia, near the Anguillara station, this type of albanose occurs. In 
the Latian District the type is common, and some localities are Monte Cavo, Grotta 
Ferrata, and the Villa Aldobrandini, near Frascati. 
Name. The name of the subrang is derived from that of the Alban Hills, in 
which this magma is represented abundantly. That of the type is derived from that 
of the region, of which this is one of the most characteristic types. 
In the prevailing systems these rocks have been uniformly called leucitites, 
and this may be regarded as their proper name, though the term should logically, it 
would seem, be reserved for rocks composed wholly of leucite or almost so.f 
ROMAL ALBANOSE. III. 8. 2. 2. 
Megascopic characters. Very dark gray or black, compact, aphyric, or with very rare 
phenocrysts of leucite and augite. Groundmass aphanitic. 
* As 4.9 per cent of the normative leucite is present in the biotite it would seem that the modal amoun 
of biotite is really somewhat more than is here calculated. 
t Cf. Cross, Am. Jour. Set., IV, 1897, p. 137; and Washington, ibid., IX, 1900, p. 54. 
