142 THE ROMAN COMAGMATIC REGION. 
In both sets as we read down there is a progressive diminution in the size and 
abundance of the phenocrysts, here assumedly all salic, in the first set of feldspar* 
and in the second of leucite. In the ischial and arsal types the feldspar phenocrysts 
are large and numerous, while in the cumal, bolsenal, and fiescolal types they are 
small and scarce, and some olivine appears in the fiescolal type. Similarly in the 
viterbal, foglianal, and vesbal types the leucite phenocrysts are large, abundant, 
and prominent, they become small and scarce in the bagnoreal and scalal, and 
there are no megaphenocrysts in the orvietal and galeral types. Types which 
resemble each other as these do, which are found in the same row, may be said to 
be texturally homologous. 
An analogous relationship is manifest modally. Thus the types of phlegrose 
show no soda-lime feldspar, those of vulsinose carry a very notable quantity of this, 
while in the types of monzonose this mineral becomes more prominent. In the 
leucitic types of ciminose soda-lime feldspar is very subordinate in amount to the 
alkali-feldspar, and there is no leucite in the groundmass; in those of vicose alkali- 
feldspar is less and soda-lime feldspar is much increased, and there is much leucite 
in the groundmass; while in the types of braccianose alkali-feldspar has disappeared, 
its place being taken by groundmass leucite, and a labradorite or anorthite is the only 
feldspar. Types which resemble each other as these do, which are found in the 
same column, may be said to be modally homologous. 
The relations are not always as simple and as clearly cut as in the above examples, 
complications being introduced by the presence of other minerals or differences in 
the minor textural characters. Thus in the above set of leucitic types there are a 
few feldspar phenocrysts and even less of augite present along with those of leucite> 
and the fabric of the groundmass, for instance, may be a trachytic one in some types 
and a felted, granular, or intersertal one in others. It will also frequently happen 
that no homologues, either modal or textural, are known. Thus the tavolatal appia- 
nose, with its very large leucite phenocrysts and abundant haliynes, does not much 
resemble any of the others, though it is texturally similar to the paglial pulaskose 
and some of the foglianal vicoses which have similarly huge leucite phenocrysts. 
Furthermore, a certain degree of latitude may be allowed in the application of 
these homologies, as is the case with the definition of types. Thus the foglianal 
ciminose-auruncose and vesbal braccianose may be said to be texturally homologous 
with the foglianal vicose described above, though the size of the leucite phenocrysts 
in these two types is smaller than is true of the last in the type specimens. 
CORRELATION WITH THE PREVAILING SYSTEMS. 
It will be clear from an examination of the table on page 14, as well as from a 
study of the descriptions, that the distinctions between the types of the quantitative 
classification are more sharply drawn in some ways than is true of those of the pre- 
vailing systems. Thus rocks which would be called vulsinite in the latter are here 
referred to at least six distinct types, being distinguished partly by their magmatic 
