PETROGRAPHY. 33 
It is clear from this that considerable soda-lime feldspar must exist among the 
groundmass feldspar laths, which is indistinguishable from those of orthoclase, partly 
due to their small size and partly to the absence of twinning, which is not always 
pronounced even in the phenocrysts of anorthite. That the groundmass soda-lime 
feldspar must be considerably more sodic than the phenocrysts is shown by their 
lower refractive index, as well as from a consideration of the norm and the composi- 
tion of the orthoclase, which can not be far from that stated above ; that is, roughly 
Or 2 Ab r . This leads to the assignment of the composition Ab x An r to the ground- 
mass plagioclase, rendering the average composition Ab T An a . From the optical 
characters of the augite it is clear that none of the acmite molecule is present; in the 
readjustment of molecules for calculating this mineral it was assumed to have the 
composition of that from the Ticchiena albanose, the analysis of which is given 
later. The amount of A1 2 O 3 thus set free by the transfer of CaO from norma- 
tive anorthite to the wollastonite demanded by the augite is just about sufficient for 
the small amount of biotite present. 
In general, the modal departures from the norm are of very small moment, so 
that the mode is a normative one, and the rock may be described as normative 
salphyro-vulsinose. 
Occurrence. The best-known locality of this type is at the town of Bolsena in 
the Vulsinian District, and it probably occurs elsewhere in the northern part of this 
district. Recently rocks of this type have been described by De Lorenzo and Riva 
from the Astroni Volcano in the Phlegrean Fields, and the same type is found in 
the Auruncan District, as in the valley east of Casi and near Torano, both near 
Teano. 
Name. The derivation of the subrang name has been already given, and that 
of the type is, of course, derived from the locality mentioned above. 
In the prevailing systems of classification rocks of this type have usually been 
designated trachyte, though they differ from the true trachytes chemically in their 
lower silica and alkalis and higher lime, and modally in the presence of consider- 
able labradorite or anorthite; and this abnormality has been commented on by 
almost every petrographer who has described them. On these grounds I proposed 
some years ago that they be separated from the true trachytes, as a separate group 
intermediate between the trachytes and basalts, and that they be called vulsinite, 
this name having been applied as well to the types known here as arsal vulsinose 
and arsal and bolsenal ciminose. While the separation of these types from the true 
trachytes and the bestowal of a distinctive name upon them has been criticized, the 
close accordance between the more numerous analyses which we now have of them, 
and their very marked dissimilarity in chemical composition from the typical 
"trachytes" of the older systems, as revealed by their norms and classification 
quantitatively, would seem to show that the separation and the use of the new 
name were amply justified. 
