100 THE ROMAN COMAGMATIC REGION. 
Chemical composition and norm as on p. 97. 
Type specimens from Poggio Cotognola and Madonna del Riposo, near Bracciano, 
Sabatinian District. 
II. 6. 2. 2. Orvietal Vicose [Leucite-Tephrite, Orvieto Type]. 
Megascopic characters. In the hand specimen these rocks are quite indistin- 
guishable from the orvietal auruncose described above. They are rather light gray 
basalts, almost, if not entirely, aphyric, the only phenocrysts being small prisms of 
dark-green augite, and even these being wanting in most specimens, and when 
present excessively scarce. The grain is distinctly phanerocrystalline, but very fine, a 
lens being needed in most cases to distinguish between the light and dark minerals. 
Microscopic characters. In thin section the texture is seen to be practically 
holocrystalline, though a very minute amount of glass may be sometimes present. 
The fabric is an intersertal one, due to the divergent arrangement of the labradorite 
laths. This labradorite, which gives extinction angles corresponding to about 
Ab 1 An I , is in thin plates, tabular parallel to b (oio), and almost always multiply 
twinned. There are some tables of alkali-feldspar as well, but these are few, and 
for the most part this mineral is anhedral and interstitial between the other com- 
ponents. Leucite is abundant, rarely in subhedral spheroids, and usually in irreg- 
ular anhedral patches, and later than the labradorite. Its double refraction is rather 
weak, and inclusions of the usual kind are not common and are seldom regularly 
arranged. There is much augite, in subhedral or anhedral prismoids and grains, of 
a greenish-gray color and non-pleochroic. This augite is often accompanied on the 
ends and sides by a later growth of a brown, pleochroic hornblende, which also 
forms separate anhedral individuals. The amount of this mineral, which may be 
referred to barkevikite, is very small. A little olivine is also seen, in small, anhedral 
grains, which is to be distinguished from the augite by its lack of color, as well as 
by its higher refractive index and birefringence. Grains of magnetite and small 
prisms of apatite are present in small quantity. There is also a very small amount 
of interstitial nephelite, with possibly a very little glass in some specimens; but the 
rock typically is holocrystalline. 
From the description just given it is clear that, with the exception of the presence 
of barkevikite, whose amount is so small as to be negligible in defining the type, 
this rock is essentially similar to the groundmass of the bagnoreal type described 
above. It may be regarded as the aphyric end of a textural series of types, of which 
the viterbal is the most porphyritic, and the bagnoreal intermediate. 
Chemical composition. The previously published analysis of this type is 
repeated here, with a number of additional determinations made recently. The 
present statement contains a correction in the amount of Na 2 O, as it was found, in 
going over the figures in the laboratory notebook, that a slip had been made in sub- 
tracting the weight of the KC1 from that of the mixed NaCl + KCl, the weight of 
NaCl being given as 0.05673 grm., instead of 0.04673 grm., the true one. 
