PETROGRAPHY. 13? 
The mode of the Ticchiena rock was likewise easy to calculate from the norm, 
but the results do not correspond as well with the microscopic appearance. This 
is due to the amount of normative akermanite present, which demands the forma- 
tion of some 1 2 per cent of melilite, and there does not appear to be nearly so much of 
this actually, though a little undoubtedly exists. There is no more normative min- 
eral present from which we can abstract any silica to change the akermanite in 
part to wollastonite to form augite, and there is no reason for thinking that the 
analysis is defective to the serious extent implied by the normative presence of so 
much akermanite and so little modal melilite. As the grain is very fine, except for 
the rather numerous augite microphenocrysts, it may well be that more melilite is 
actually present than would appear in the section. However this may be, the results 
of the calculation are as follows : 
Anorthite 3.8 
Leucite 39-7 
Nephelite 9.8 
Augite 28 . 9 
Olivine 1.6 
Melilite 12.2 
Ores 3.0 
Apatite i . o 
100.0 
This mode corresponds well with the thin section as far as the amounts of an- 
orthite, leucite, olivine, ores, and apatite go, and that of augite is fairly close, though 
the apparent amount is somewhat greater. The main variations from the thin sec- 
tion are in the higher nephelite and melilite, the latter of which has been spoken of 
above. The mode is a normative one on the whole, but the implied presence of so 
much melilite, even though discoverable with difficulty, would suggest that this 
rock be distinguished from that of Pofi. This could be done by erecting it into a 
distinct type, whch might be called the ticchienal, and distinguished from the true 
saccal by the presence of melilite, and from the boval type to be described next, 
which is also rich in melilite, by the phenocrystic character of the augite and the less 
obvious melilite. But to avoid an undue number of type names this rock will, for 
the present, be considered to be of the same type as the Pofi one, that is, saccal. 
Occurrence. This type of albanose is by no means so common as the romal, 
and specimens could be identified with certainty only from the Hernican District, 
at the two localities cited, from the Vulsinian, where it forms the flow on the summit 
of Monte Rado, above that mentioned on pp. 131 and 132, and in the Sabatinian 
at the Fosso San Celso, on the south shore of Lake Bracciano. 
Name. The derivation of the subrang name has been already discussed. The 
typal adjective is derived from the Sacco River, in the valley of which rocks of this 
type are prominent. 
In the prevailing classifications these rocks are for the most part to be regarded 
as leucitites, though the small amount of plagioclase in some specimens has suggested 
the name of leucite-tephrite as the more appropriate. The smallness of the quantity 
of olivine would render the term leucite-basanite inappropriate. 
