5 2 
THE ROMAN COMAGMATIC REGION. 
Norm of I. 
Or 
,Q Q2 ) 
48.09 
2.20 
S.io 1 
3.61 
86.43 
1 
12.05 
An . 
6 12 S 
Lc .... 
28 78 ) 
\ T e . ... 
. ( 
HI 
Q f \ 
Th 
i-8<; ( 
Di .... 
3.46 > 
Wo 
4.64 ) 
Mt 
2. CC ) 
11 
1 . 06 ) 
Ap... 
O. 74. 
Class 
Ratios of I. 
Sal 
' 'Fern 
= 7.17 
Order. 
Rang 
Subrang. 
F 
'L 
K 2 O' + Na 2 O' 
CaO' 
K a O' 
= 0.72 
= 9.28 
'Na 2 O' 
= i-43 
Rest, 
i-43 
99.91 
The magmatic position of the tavolatal appianose is distinctly transitional as 
regards class, the ratio of sal to fern being almost exactly on the border between per- 
salane and dosalane. The magma consequently is a janeirose-appianose. The 
similarity to an analysis of a typical janeirose rock is shown by comparison of I and 
IV, the chief divergencies being in the alkalis. It will be seen on reference to the 
norms of the two that the dosalic position of the latter rock is brought about by the 
presence of acmite, both normative and modal, while in the Italian rock this mineral 
does not occur normatively, and only to a subordinate extent modally. As regards 
order, rang, and subrang, it need only be said that the tavolatal appianose is well 
within the respective limits of these divisions. 
The only other analysis yet known of an appianose rock is one from Magnet 
Cove, shown in III, which is non-leucitic, the only lenad present being nephelite. 
As the alkalis show, this latter rock is quite close to the dosodic subrang laugenose. 
Mode. Owing to the small size and confused arrangement of the groundmass 
constituents and the presence of the ill-defined cement, an exact estimation of the 
mode by Rosiwal's method was almost impossible, though it was practicable for 
some of the constituents, as the phenocrysts of leucite, labradorite, haiiyne, aegirite- 
augite, and melanite. The complexity of the mineral composition also precluded 
an exact calculation from the norm. By assuming that the composition of the 
pyroxene was that of the asgirite-augite of Elfdalen, and that those of the haiiyne 
and melanite were represented by well-known analyses of these minerals from the 
Alban Hills, and checking the calculations by the microscopic observations, the 
following mode was estimated. That it is approximately correct is shown by a com- 
parison of the chemical composition recalculated from this with that obtained by 
chemical analysis. The agreement is seen to be as satisfactory as could have been 
expected when dealing with such a complex rock. 
Orthoclase 19.2 
Labradorite, Ab x An 2 7.0 
Leucite 37 
Nephelite 11.7 
Haiiyne 10 . o 
^Egirite-augite 11.5 
Melanite 2.0 
Biotite 1.3 
Apatite 0.3 
