io8 THE ROMAN COMAGMATIC REGION. 
Name. The type derives its name from Monte Somma, where it occurs abun- 
dantly. In the prevailing systems of classification it would be called a leucite- 
tephrite, the small amount of olivine being regarded as negligible. 
II. 7. 2. 2. Qaleral Braccianose [Leucitite, Galera Type]. 
Field characters. This type is very dark, compact and heavy. It is only 
slightly porphyritic megascopically or aphyric, phenocrysts of leucite making up 
less than 5 per cent of the rock volume and sometimes being absent. They are 
usually well-formed trapezohedra, often fragmentary, white or very pale gray, and 
often with a slightly waxy luster, and vary in diameter from 5 to 10 mm. Small 
phenocrysts of augite are either wholly wanting (the typical case) or are present in 
extremely small quantities. No other phenocrysts are found. The groundmass is 
a typical basaltic one, very dark gray or almost black, fine-grained, and aphanitic, 
but without a vitreous luster. Some of the specimens show a few vesicles, but the 
majority are quite free from them. The type would be called a leucitic melaphyre 
or a basalt in the field. 
Microscopic characters. The very few large leucites seen in the thin sections 
show the usual characters, good development of the crystalline form, often fragmen- 
tary, pronounced birefringence, and comparatively few inclusions. The still rarer 
large augite phenocrysts call for no remark, as they are of the type common in the 
region, and already often described. 
The groundmass is percrystalline and microporphyritic, and shows a fabric 
which is highly characteristic of many of the more femic leucite rocks of the region. 
Leucite microphenocrysts are abundant, making up about one-third of the rock. 
They present rounded circular sections, and are usually anhedral, though occa- 
sional crystal planes are seen. Their size is fairly uniform, mostly between 0.20 
and 0.30 mm. in diameter, though smaller and slightly larger individuals occur. 
Double refraction is faint, but readily seen with the selenite plate. Inclusions are 
apt to be few, but some specimens show many small ones, of glass, augite, or mag- 
netite, which usually form a circular ring near the periphery of the crystal, or else 
are arranged so as to show a skeletal development of the leucite. Rarely the 
original skeleton forms are well preserved. 
Between these leucites is the microgroundmass, which consists in great part of 
a felted mass of very minute, pale, greenish-gray prisms of augite, with occasional 
larger individuals, which are to be regarded as microphenocrysts. While the gen- 
eral arrangement of these small prisms is diverse, adjacent to the leucites they 
are tangential, as if they had been present in the still molten magma prior to the 
complete crystallization of the salic mineral and had been shoved aside during its 
growth. With these augites are small grains of magnetite, as well as some laths of 
feldspar, which is mostly a labradorite. 
Acting as a cement for these crystals is a clear, colorless substance, the amount 
of which is very difficult to estimate on account of the close juxtaposition and mostly 
