90 THE ROMAN COMAGMATIC REGION. 
silica thus set free in the formation of biotite from the more highly silicated minerals 
hypersthene and orthoclase, having nothing to combine with, must be present as 
quartz or as a quaric glass. 
Quartz. . . 6.8 
Orthoclase, Or 2 Ab t 33-3 
Labradorite, Ab 2 An 3 34 . i 
Augite 12. i 
Biotite 7.3 
Ores 6.0 
Apatite 0.4 
From the above it is clear that a certain amount of quartz must be present in 
the rock, and this is to be looked for in the rather ill-defined, colorless, anisotropic, 
interstitial base already mentioned. The feldspar laths and what are clearly anhedra 
of orthoclase are so closely juxtaposed that study of this base is difficult, but the 
mean index of refraction appears to lie between those of orthoclase and labradorite 
and the rock does not furnish any gelatinous silica on treatment with acid. The 
mode as thus calculated corresponds with the thin sections of the rock as well as 
can be estimated, and may be considered as close to the truth. 
The variation of the mode from the norm appears chiefly in the presence of 
biotite, the other readjustments being of minor importance. The type may there- 
fore be described as a biotite trachiphyro-shoshonose. 
Occurrence. This type is a decidedly rare one, occurring only in the Auruncan 
District, and here only at the central dome of Monte Santa Croce, which is com- 
posed apparently entirely of it, and at the adjoining small hill of Monte Lattani in 
an altered condition. 
Name. The name of the subrang is derived from that of Iddings's group of 
shoshonites, many of which fall here. The type name is derived from that of the 
volcano of Rocca Monfina, itself so called from the name of the village at the foot 
of Monte Santa Croce, where the type occurs. 
In the prevailing systems of classification the type has been assigned to very 
different positions. Called a trachydolerite by Abich, as far back as 1841, it is 
considered by Pilla, vom Rath, and Roth to be a trachyte, while Bucca regards it as 
an augite-andesite. In a former description I bestowed the name of biotite- vulsinite 
upon it, to indicate at the same time its chemical relation to the vulsinites and its 
mineralogical divergence; and this would seem to be an appropriate name, though 
it would best be called a biotite-latite. 
MONFINAL SHOSHONOSE. II. 5. 3. 3. 
Megascopic characters. Light gray, sometimes with pinkish tinge, compact, porphyritic. 
Augite phenocrysts common, black or greenish-black, small prisms. Biotite phenocrysts, com- 
mon and conspicuous, black, glistening, small tables. Feldspar phenocrysts few, small, white, 
prismatic, inconspicuous. Groundmass, fine-grained, phanerocrystalline. 
Specific gravity = 2.717. 
