134 THE ROMAN COMAGMATIC REGION. 
naturally dissolve all the leucite, olivine, melilite, magnetite, apatite, and anorthite, 
which are the only other minerals present, and would have only a slight effect on the 
augite, removing the outer portions. A few larger augites in the specimen were 
zonal, but the majority of them and all the small ones were quite uniform in char- 
acter from center to periphery. The analysis resulted as follows: 
Analysis of Augite (Fedorowite) from Ticchiena, Hernican District. 
Si0 2 45-46 0.758 
TiO 2 2.85 .036 
A1 2 O 3 6.47 .063 
Fe 2 O 3 6.62 .041 
FeO 5 . 64 .078 
MnO 1 5 0.002 
MgO 12.72 . 293 
CaO 20.17 360 
Na 2 O 0.72 .012 
K 2 O o . 65 .007 
100.45 
Combining these into the respective amounts of pyroxene molecules, we get the 
following percentage composition : 
(Mg, Fe)(Al, Fe) 2 SiO 6 21.00 
CaMgSijOe 62 . 21 
CaSiOj 8.35 
(Na, K)FcSi 2 O 6 9.00 
100.56 
That is, the pyroxene has the molecular composition 85 (Mg, FeAl, Fe) 2 
SiO 6 .288CaMgSi 2 O 6 .72CaSi 2 O 3 .19(Na, K) FeSi 2 O 6 , or more roughly 4 (Ca,Mg) 
SiO 3 + (Mg, Fe) (Al, Fe) 2 Si 2 O6 with less than 1:20 of the acmite molecule. Cal- 
culating the norm of this pyroxene as if it were a rock, we get: 
Or . . ... 
2 . =;o 
Wo 
. 2.67 
An 
. 12.23 
Mt 
. O. "\I 
Lc 
I OQ 
11 
^ .47 
Ne 
Di.. 
. 6?. 2O 
JOO. 17 
It is thus seen that salic constituents make up about one-fifth of the augite, and 
may thus introduce very notable differences between the norm and mode in many 
cases. 
It may be mentioned, by the way, that the norm calculated above was used in 
readjusting the figures of the norm to calculate the mode a, as this procedure was 
found to be more easy and simple than that of ratios of the different chemical con- 
stituents as compared with the data in the assumed mineral composition, as ex- 
plained elsewhere.* 
Between the larger augites, and either between or inclosing the smaller ones, are 
small formless or sometimes rounded individuals of leucite, which acts as an intersti- 
tial cement to a large extent, though the individuals are small and not poikilitic in 
the usual sense of the term. With these leucites are small areas of melilite in most 
* Cross, Iddings, Pirsson, Washington, Quant, Class. Ign. Rocks, Chicago, 1003, pp. 216 ff. 
