THE ROMAN COMAGMATIC REGION. 
Biotite. About 4 per cent, 0.05 to o.iomm., anhedral, irregular patches, pale brown, 
pleochroic. 
Magnetite. About 4 per cent, 0.02 to 0.05 mm., anhedral, equant. 
Apatite. About i per cent, 0.05 to o.io mm., subhedral, prismatic. 
Chemical composition and norm as on p. 86. 
Type specimen from below Toscanella, Vulsinian District. 
II. 5. 3. 3. Monfinal Shoshonose [ Biotite- Latite, Monfina Type]. 
Megascopic characters. This rock, which is only met with in the region as a 
domal eruption, is highly porphyritic, showing many small, glistening, black tables 
of biotite and small greenish-black augite prisms, with numerous, but much less 
conspicuous, small, stout prisms of feldspar in a very fine-grained, phanerocrystalline 
groundmass, which is very light gray, but with a slightly pinkish tone. The field 
name for the type would be biotite leucophyre. 
Microscopic characters. The most abundant phenocrysts are alferric, of a 
greenish-brown biotite in thin tables, which are fresh in the interior but with a thin 
border of alteration products, and of subhedral, stout prismoids of the usual augite, 
though its color is apt to be slightly more green than in the types just described. 
Phenocrysts of feldspar are rather less numerous, though common, and consist of 
both labradorite (Ab 2 An 3 ) and soda-orthoclase. These are subhedral, stout prisms, 
whose average lengths, like those of the alferric phenocrysts, run from 0.5 to i mm., 
seldom being greater than 2 mm. 
The groundmass in which these lie is very largely composed of slender laths of 
feldspar, here mostly soda-orthoclase, with less labradorite, these being arranged 
fluidally, so that the fabric is a typical trachytic one. With them are small augite 
prismoids, small magnetite anhedra, and very few apatite needles. All these lie in 
a clear, colorless base, which is not isotropic, but which everywhere shows a distinct, 
though somewhat faint, birefringence. As will be seen later, when the norm and 
the mode are discussed, this base is in all probability either quartz or a mixture of 
quartz and alkali-feldspar. 
Chemical composition. An analysis of this type, made several years ago, is 
here repeated with additional determinations. For comparison there is also given 
an analysis of another type of shoshonose from Radicofani, the magma of which is 
probably connected with the Vulsinian District. 
Chemical Composition of Monfinal Shoshonose \Biotite-latite}. 
I. 
ii. 
I. 
n. 
SiO 2 .... 
H 2 O + 
O 17 
O. I? 
A1 2 O 3 
17 8? .17^ 
i 6 40 . 162 
CO 2 . . 
none 
none 
Fe 2 O,... 
A O7 O2^ 
i 02 006 
TiO 2 
i . 02 o. 013 
i . 10 o. 014 
FeO 
326 O4C 
c 6s 078 
p,O. 
o. 10 . ooi 
n.d. 
MgO 
7 AT 08 C 
8 S7 214. 
MnO 
n d 
n.d. 
PaO 
Na 2 O 
.07 .123 
2.89 .047 
7-95 I 42 
2.07 .033 
99-85 
100.91 
K 2 O 
7 7? .O?6 
SD srr . 
2 . 717 
