PETROGRAPHY. 107 
II. 7. 2. 2. Sommal Braccianose [Leucite-Tephrite, Somma Type]. 
Megascopic characters. Like the preceding type, this is highly porphyritic. 
The phenocrysts are of both leucite and augite, present in about equal amount and 
together making up about 40 per cent or so of the rock volume. Those of leucite 
are from i to 4 mm. in diameter, rounded, and anhedral, with only occasional crystal 
planes. Their color is usually gray, so that they do not stand out prominently 
against the groundmass. The phenocrysts of augite are about the same size, usually 
subhedral and with crystal planes, especially those of the prisms and pinacoids. 
In form they tend to be equant rather than prismatic, giving square or only slightly 
oblong and angular sections. They are very dark green in color, almost black. An 
occasional yellowish-green grain of olivine is seen, but the amount of this mineral 
is negligible. The groundmass is dark gray and aphanitic. In the field the type 
would be called a leucite-augite-melaphyre. 
Microscopic characters. The thin sections of this type present much the same 
appearance as those of the vesbal. There are the same rounded leucite phenocrysts, 
showing feeble double refraction and carrying few inclusions, but they are not 
nearly as thickly scattered as in the preceding type. On the other hand, pheno- 
crysts of pale-gray augite are much more abundant, and one or two subhedral olivine 
phenocrysts are seen in the section. 
The groundmass between these is like the preceding, especially in the plagio- 
clase laths and their diverse arrangement, giving rise to a somewhat intersertal fabric. 
Leucite anhedra are common, this mineral often forming irregular interstitial areas. 
Small grains of augite are abundant, though less so than in the preceding type, as 
much of this mineral is here developed phenocrystically. There are also small 
anhedra of olivine and considerable residual base, which may be of glass, but is 
nephelite for the most part. The usual magnetite grains and apatite prisms may 
also be mentioned. 
Chemical composition and mode. No'analysis has been made of a rock of this 
type, so that it is uncertain whether the classificatory position assigned to it is correct 
or not. As, however, all the other Vesuvian lavas analyzed by me, and many of 
those analyzed by others, fall in braccianose, there is little room for doubt that 
some, if not most, of the rocks with this mode and texture are really in this subrang. 
The mode could not be satisfactorily estimated by microscopic methods, but 
the study of the thin sections indicates clearly that it must be quite similar to that 
of the vesbal type, though possibly with slightly greater relative amounts of alferric 
minerals. 
Occurrence. The most typical occurrence of this rock is the flow of 1754 near 
Bosco Reale at Mount Vesuvius. Other flows which belong to this type, as deter- 
mined by specimens examined by me, are those of 1750, 1751, 1771, 1834, and 1858, 
while the descriptions of Fuchs lead one to refer here as well those of 1809, 1832, 
and 1839. The type is also abundant at the encircling Monte Somma, both in the 
lava flows and the dikes. 
