158 THE ROMAN COMAGMATIC REGION. 
The composition would seem to be very uniform and may be represented by 
the analysis on p. 134, and it is rather a diopside augite than one with the heden- 
bergite molecule abundant. In some, but not all, of the rocks belonging to sodi- 
potassic subrangs the augite carries some aegirite, but the amount of this is usually 
small, and aegirite-augite must be regarded as rare or not prominent in the region. 
The color of the augite is one of its most constant and characteristic features- 
While black or very dark green in the hand specimen, in thin sections it is almost 
invariably either colorless or a very pale grayish, occasionally slightly tinged with 
green or yellow. Only in some of the sodipotassic rocks does the color assume a 
true green, when it is pleochroic, and evidently contains some aegirite. But in the 
very great majority of the rocks, whether persalanes, dosalanes, or salfemanes, the 
typical color is the very pale gray, which may be said to be characteristic. 
In this connection it is worthy of remark that, though the magmas of the region 
are rather high in titanium, and the augite analyzed by me contains 2 .85 per cent 
of titanium dioxide, this mineral never shows the peculiar purple tones which are 
commonly supposed to be due to this oxide. A somewhat similar state of affairs 
has been observed by Pirsson in the Central Montana Region, as will be brought 
out later. 
Hypersthene. This mineral is one of the rarities of the region, and its absence, 
rather than its occurrence, is to be regarded as characteristic. It was only observed 
in the quaric and vitreous sorianal harzose, in colorless crystals, but is common in 
the quaric types of toscanose (I. 4. 2. 3) and amiatose (I. 4. 3. 3) at Monte Amiata, 
to the north of the Vulsinian District, the magmas of which may be connected with 
those of the Roman Region. 
Biotite. This mineral is likewise rare in the region, though more common 
than hypersthene. In some types, as sorianal harzose (II. 4. 3. 3) [biotite-latite], 
teanal auruncose (II. 5. 3. 2) [biotitic leucite- trachyte], and monfinal shoshonose 
(II. 5. 3. 3) [biotite-vulsinite], all of which, it is to be noted, are alkalicalcic, it is 
quite abundant and an essential component. In other types, as arsal vulsinose, 
ciminose (II. 5. 2. 2), and monzonose (II. 5. 2. 3) [vulsinite], it occurs in accessory 
amounts, and mostly as small interstitial patches. It is also met with, though as 
a rare accessory, in some of the types of braccianose (II. 7. 2. 2) at the Vesuvius 
Volcano. The color is uniformly a pale brown, with marked pleochroism. 
There would seem to be some connection between the occurrence of biotite 
and that of olivine, the potential biotite molecule splitting up into leucite and olivine. 
A number of instances of this are mentioned in the descriptions of the types, and 
the subject has been discussed by the writer and other petrographers. On the 
whole the rarity of biotite, rather than its occurrence, may be regarded as the 
feature in regard to it which is most characteristic of the region. 
Olivine. Notwithstanding the rather low silica of many of the rocks this 
mineral is decidedly uncommon, though it is met with in small amounts in many of 
the dosalanes and salfemanes, and is characteristic of fiescolal ciminose (II. 5. 
