COMPARISON WITH OTHER REGIONS. 
Magmas 0} Roman, Campanian, and Central Montana Regions. 
179 
AVERAGE ROMAN 
MAGMA. 
AVERAGE CAM- 
PANIAN MAGMA. 
AVERAGE HIGH- 
WOOD MAGMA. 
SiOj 
e?.4 
C4 O 
C2 2 
A1 2 O 3 
18.3 
18 2 
T C 3 
Fe,O,. 
2.Q 
2 6 
FeO 
3 A 
MgO 
J . I 
2 7 
CaO 
6.4 
e 6 
7 8 
Na 2 O 
7.0 
4-7 
K 2 O 
8.? 
7 5 
6 i 
TiO 2 
0.8 
o 8 
P 2 O< 
O.4 
O 4 
IOO.O 
IOO.O 
97.6 
(II. 6. 2. 2) 
(II. 6. 2. 3) 
II.6.2.3) 
In general features the resemblance between the Italian and the Montana 
magmas is close, though it must be remembered that in III we are not dealing with 
the magma of the whole region, but with that of a single district. It is clear that 
the last is distinctly higher in the femic constituents, the two oxides of iron, mag- 
nesia, and lime, and lower in alumina and potash, silica and soda being about the 
same in both. 
Mineralogically also there are some features in common. One of the most 
striking is the occurrence of many leucite-rich types in both regions, though these 
are much more abundant in the Roman Region. This is undoubtedly connected 
with the more potassic character of its magmas, and possibly also with the difference 
in the geologic occurrences of the rocks. Another is the predominance in both 
regions of augite as the alferric mineral. The color and other physical properties 
differ somewhat, it is true, but in both cases this mineral, though titaniferous, 
does not show the purple tones found elsewhere, and the constant occurrence of 
a diopside-augite in two comagmatic regions so closely alike magmatically is certainly 
a very striking fact, and possibly one of some significance. 
Again, while biotite is rather more common in the Montana Region, possibly 
because of the intrusive nature of most of the Montana rocks, it is not an abundant 
mineral in either, while the hornblendes are rare in both regions. Similarly the 
alkali-feldspar is invariably a soda-orthoclase, and microcline and microperthitic 
intergrowths are equally rare in both. 
These comparisons could be carried further and with greater detail, but the 
above must suffice for the present. It shows clearly that, while there are certain 
constant differences, there is a very great degree of similarity between the two 
regions, not only in the absolute and serial chemical and the mineralogical charac- 
ters, but in the distribution in space of the various magmas and rock types. 
The bearing of these and similar facts upon the general questions of differen- 
tiation, its causes, conditions, and processes, is as yet very obscure. As Pirsson 
