PETROGRAPHY. 13 
regard to leucite-trachyte and leucite-phonolite has been followed rather than that 
of Rosenbusch, for reasons discussed elsewhere. The term "leucite-trachyte," 
therefore, as used in this paper, implies a rock composed essentially of leucite and 
orthoclase, while a "leucite-phonolite" is one composed of the same two minerals 
with nephelite in addition, in both cases the presence of subordinate amounts of 
alferric and other minerals being allowed. It has sometimes been difficult to decide 
between two names, as, for instance, between "leucite-trachyte" and "leucite- 
tephrite," or " leucite-tephrite " and "leucitite," as both labradorite and ortho- 
clase may be present in the former case, or a little labradorite alone in the latter, in 
both cases with much leucite. In all such cases the name finally selected was based, 
as far as possible, on the quantitative estimation of the mode, modified in some 
instances by the greater prominence of one or the other mineral. 
A list is appended of the various rock types which have been observed and 
described in the region, with their corresponding names according to the prevailing 
systems, so that comparison between the two may be rendered more easy. In the 
prevailing names I have followed the trend of some modern authors in further quali- 
fying the broader and more common names by the use of type names. This would 
seem to be reasonable, as it is certainly advisable to be able to discriminate, even in 
the customary nomenclature, between, for instance, three such different "leucite- 
tephrites" as those of the Viterbo type, with its large and numerous leucite pheno- 
crysts, that of the Orvieto type, without any phenocrysts, and that of the Tavolato 
type, with large leucite phenocrysts, abundant hatiyne, little feldspar, and very 
different chemical composition. 
In two or three instances new names have been proposed for use in the pre- 
vailing system of nomenclature. This has been done with caution, and only in those 
cases where they seemed to be especially necessary or useful, on account of the pecu- 
liar chemical, mineralogical, or textural character of the type, as with the abundant 
and highly characteristic leucite-trachytes and leucite-tephrites with the viterboid 
habit marked by the numerous large leucite phenocrysts, or the peculiar leucite- 
tephrite of Tavolato, with its abundant large hauynes and peculiar chemical com- 
position, which has little in common with the leucite-tephrites as usually understood. 
In one instance an old and forgotten rock name has been revived that of cecilite 
(Cordier), for the highly melilitic leucitites. 
It is seen that many of the prevailing rock names cover wide variations in tex- 
ture and magmatic characters a point which will be more fully appreciated by 
referring to the descriptions which follow. And it is also evident that some of the 
subrang names are applied to rocks which differ much in their modes and textures, 
a consequence of the fact that the quantitative system is based primarily on the 
chemical composition of the rock, and that the modal and textural characters are 
regarded as of less importance, they being expressed by the type name. A study of 
the table from this point of view is very instructive, and the relation between the two 
systems should be borne in mind when reading the descriptions of the various types. 
