156 THE ROMAN COMAGMATIC REGION. 
characters. In addition to the presence of certain minerals and their peculiarities, 
the absence of others which might be expected to be present, so far as the magmatic 
or normative characters permit, is another mineralogical characteristic of comag- 
matic regions. 
Soda-orthoclase. A constant feature of the alkali-feldspars of the region is 
that pure orthoclase seems never to occur, notwithstanding the highly potassic 
character of the magmas, the orthoclase molecule being invariably accompanied 
by notable amounts of albite, so that the alkali-feldspar is a soda-orthoclase. The 
orthoclase molecule, however, is nearly always present in larger amount than that 
of albite, the composition varying from Or6Ab x to (rarely) OrjAbj. As a common 
character it is notable that the microcline structure is never seen, and microper- 
thitic intergrowths of orthoclase and albite are exceptional, while zonal structure 
is uncommon, the sections of the soda-orthoclase usually appearing uniform and 
homogeneous. 
In habit the soda-orthoclase occurs either as phenocrysts, often of large size* 
and either prismatic, parallel to the clinoaxis a, or tabular, parallel to the clino- 
pinacoid b (oio), or as small prismatic laths (parallel to the axis a) in the ground- 
mass. It may also rarely form an interstitial cement, and in some types occurs 
thus as a mantle of later growth around phenocrysts of soda-orthoclase or soda- 
lime feldspar and oriented parallel with them. 
It occurs most abundantly in the persalanes, especially in the phlegrose rocks, 
is common in the dosalanes, except in braccianose, where it is wanting, and is not 
found in the salfemanes. Its presence in the mode, therefore, would seem to be 
directly connected with the variations in the silica and the femic components. 
Soda-lime feldspar. The composition of this varies considerably, from ande- 
sine, Ab 3 An 2 , to an almost or wholly pure anorthite, but in the majority of the 
types it approximates to a labradorite, AbjAn 2 . Multiple twinning is common, 
except in the groundmass laths of some types, according to the Carlsbad and albite, 
and less often the pericline laws. Zonal structure is not frequent, but, as said 
above, in some types phenocrysts of labradorite or anorthite are surrounded by 
mantles of orthoclase oriented like the nuclear crystal, that is, with the vertical 
axes and the clinopinacoids b (oio) parallel. 
The habits of these feldspars resemble those of the soda-orthoclase, but it 
may be noted that the phenocrysts of plagioclase are usually smaller than those of 
orthoclase, they are apt to be clustered into groups, and that groundmass laths are 
less common. These feldspars are rarely known to occur as an interstitial cement. 
The modal occurrence of the soda-lime feldspars is exactly the opposite of 
that observed in the case of the soda-orthoclase. They are wanting in rocks belong- 
ing to phlegrose and other peralkalic subrangs of persalane, rare generally in the 
rocks of the persalane class, most common in the dosalanes, especially in braccian- 
ose, where they constitute the only feldspars, but, like the soda-orthoclase, are usually 
absent from the salfemic rocks. 
