PETROGRAPHY. 
53 
I. 
II. 
I. 
II. 
SiO 2 
cx.o 
"?o. ? 
K 2 O 
ii 8 
Al a O 3 
21.7 
21.4 
TiO 2 
XI -3 
o 6 
Fe 2 O 3 
2-4 
1.8 
P 2 Oc . 
FeO 
O.7 
1.8 
SO, . 
MgO 
0.8 
o. ? 
Cl 
O I 
CaO 
4e 
Na 2 O 
C. I 
C.2 
100. I 
98.6 
I. Calculated from the mode. II. Determined by chemical analysis. 
Comparison of this mode with the norm shows that, while the divergencies are 
considerable, the readjustments affect all of the normative molecules and are well 
distributed. The readjustments of most moment are the formation of the haiiyne 
from normative nephelite, brought about by the normative thenardite and halite, 
and the large increase in the amount of leucite, chiefly brought about by the change 
of some of the normative nephelite to modal albite in the lab rado rite. Readjust- 
ments of subsidiary importance are those involved in the formation of the aegirite- 
augite and melanite, the latter being the most striking varietal mineral, as the pres- 
ence of some modal acmite molecules are to be expected in sodipotassic rocks of this 
region. Taking all these into consideration, as well as the fact that the pheno- 
crysts are of the lenads leucite and haiiyne, we may describe the Tavolato blocks as 
melanitic haiiyne-lenphyro-janeirose-appianose, a rather ungainly nomenclatorial 
product, which the shorter tavolatal appianose replaces advantageously. 
Occurrence. This type is represented only in the Latian District, and here but 
sparingly. The best-known locality is that near the Osteria di Tavolato, on the 
Via Appia Nuova, about 5 km. from Rome, where it is found as blocks in an agglom- 
erate of volcanic material, along with rocks belonging to the types and subrangs 
which are more common in the district. The provenience of these Tavolato blocks 
is, as yet, uncertain, although they are undoubtedly derived from the Albano Vol- 
cano. Closely similar types have been described by Struever and Sabatini from 
elsewhere in the district, as near the Lago di Nemi. 
Name. The name of the subrang is derived from that of the old Roman high- 
way, the Via Appia, near which the type occurs. The type adjective is derived 
from the oldest and best-known locality named above. 
In the prevailing classifications this type is constantly called a leucite-tephrite, 
of the phonolithoid Typus according to Rosenbusch. But the classification of these 
rocks as tephrites is scarcely justified by the mode described above, by which it is seen 
that the rock contains only about 7 per cent of plagioclase, but about 19 of orthoclase 
and 12 of nephelite, and this subordinate position of the lime-soda feldspar is more 
or less clearly evident in the descriptions published by others. Disregarding this 
mineral, then, and taking into consideration the very notable amounts of the lenads 
nephelite and haiiyne, the type should be called a haiiynitic leucite-phonolite in the 
sense of Zirkel, or a leucitophyr of Rosenbusch. The remarkable chemical com- 
