94 THE ROMAN COMAGMATIC REGION. 
The two modes are closely alike in all respects, except as to the relative amounts 
of the two feldspars. Indeed, the divergence is so great that they should strictly be 
regarded as two distinct types. But considerations of the inadvisability of making 
the types too numerous at this stage has resulted in the final decision to throw them 
both together, leaving their separation to the future if the need for this is felt. It 
is, however, to be noted that the difference in mode shown above is evident in the 
thin sections, those of the Auruncan rock (II) showing more abundant labradorite 
than the other. 
The composition of the alkali-feldspar is a striking feature, the albite molecule 
in each case being double that of the orthoclase. This is a decidedly unusual com- 
position for the alkali-feldspars of the region, which usually have the orthoclase 
molecule predominant over that of albite. It is difficult to check this calculated 
composition by optical means, on account of the small size of the groundmass laths, 
the absence of twinning, and the frequency of formless anhedra, but such measure- 
ments as were made would seem to confirm it, and it will be remembered that the 
larger alkali-feldspars in these rocks show the moire appearance which is unusual 
elsewhere in this region. 
As compared with the norm, the only difference of importance is the replace- 
ment of normative nephelite by modal leucite, and the consequent changes in the 
figures for orthoclase and albite. The type may then be described as leucite sal- 
phyro-vicose. 
Occurrence. This type is a rather common one, being found in the four largest 
districts. In the Vulsinian District the most prominent locality is a flow above 
Santa Trinita 1 , near Orvieto, but not the one whence came the specimen described 
by Klein.* In the Ciminian District the type is especially abundant, along with 
the homologous types of vulsinose and ciminose, among the products of the Vico 
Volcano, as in the inner north wall, the Contrada di Merlano and the Villa di 
Buonrespiro on the northwestern slope, and as flows from Monte Fogliano on the 
western. In the Sabatinian District it was found at Lagosello, northeast of Lake 
Bracciano. In the Auruncan District it is quite common, specimens having been 
obtained by me from Monte San Antonio, on the north wall, and occurring as well 
probably at Colle Friello, Fontanelle, and above San Martino, to judge from the 
descriptions of Bucca. 
Name. The name of the subrang is derived from that of the Vico Volcano, 
where the magma is quite abundant and where one of the most prominent types 
occurs. The type adjective is derived from the name of the highest point of the 
Vico crater, Monte Fogliano, where the type occurs. 
In the prevailing systems this type has been very variously classified, accord- 
ing to the personal bias as to the relative importance of the salic minerals, the names 
leucite-trachyte, leucite-tephrite, leucite-phonolite, and leucitophyre having been 
bestowed upon it by different authors, and the name of leucite-basanite being also 
* Klein, Neu. Jahrb., B. B. VI, 1889, p. 19. 
