PETROGRAPHY. 45 
leucite and only 5.6 per cent of orthoclase. These amounts are obviously incorrect, 
as the amount of leucite shown by Rosiwal's method can not be so far from the truth, 
since this mineral is mostly in the form of phenocrysts and is easily measurable. If 
we then assume that the amount of leucite is that given above 16.6 per cent and that 
all the olivine molecules enter as hypersthene into the augite, we shall obtain the 
following results: 
Orthoclase, Or s Ab 3 48. 3 
Labradorite, Ab t An 2 18.3 
Leucite 16.8 
Nephelite 6.5 
Augite 6.2 
Ores 3.9 
IOO.O 
This calculated mode may be regarded as closely correct, though it differs from 
the microscopic description in the presence of nephelite. The amount of this, 
however, is small, and it could easily be overlooked among the closely packed feld- 
spar laths. After attention was directed to its probable presence, examination under 
high powers revealed some colorless interstitial cement, with weak birefringence, 
which may well be this mineral. 
The deviation of the mode from the norm is therefore considerable, but is 
especially marked in the presence of abundant leucite, replacing some of the norma- 
tive orthoclase and nephelite, much of the latter becoming albite and entering the 
feldspars. The readjustments due to alferric minerals are negligible. The rock 
may therefore be described as a leucite salphyro-procenose-pulaskose. 
Occurrence. The most important occurrence of the type of paglial procenose- 
pulaskose is the very extensive flow which, issuing from the Latera Volcano in the 
northwestern part of the Vulsinian District, flowed north and ended at the Paglia 
River, near Proceno and Acquapendente. The specimen analyzed came from below 
the castle of Proceno, but specimens of the same type were obtained by me from 
elsewhere in the flow. From the descriptions of Bucca and Klein it is probable 
that the same type, or similar ones, occur at Gradoli, Monte Calveglia, Poggio Pi- 
lato, and elsewhere around the Latera Volcano, though I can not confirm this supposi- 
tion definitely by actual specimens, as I was prevented from visiting these localities. 
The same type was also found by me at Monte Bisenzo, on the southwest shore of 
Lake Bolsena, though the majority of the rocks of this locality belong to other types 
and magmas. 
Name. The derivations of the subrang names have been explained elsewhere, 
and that of the type is derived from the Paglia River, at which the flow composed 
of the type ends the most prominent geographical feature of the neighborhood. 
The rocks of this type are nearly always referred to as leucite-tephrite by the 
Italian and German petrographers, on account of their content in labradorite. Such 
a designation, however, exaggerates the importance of this constituent at the 
expense of the very much more abundant orthoclase. Furthermore, these rocks, 
with their large leucite phenocrysts and rather light-gray groundmass and abundant 
