6 4 
THE ROMAN COMAGMATIC REGION. 
Norms. 
I. 
II. 
0.30 
Q none o . 30 
Or 39-48) 37.81) ' [63.82 
Ab 15-20 J 66.36 11.53 [ 63.52 \ 
An 11.68) 14.18) 
Di 14 89 ) 14 4 6 ) 
Hy 3-59 ^29.46 18.92 ^33.38 
Ol 10. 98 ) none ) 
Mt 1.86 33.30 1.86 6 36.67 
II I.06J 2 '9 2 0.76$ 2 ' 02 
Ap 0.92 0.92 0.67 0.67 
99 . 66 i oo . 49 
Rest 0.38 0.18 
100.04 100.67 
Ratios. 
1. II. 
Sal 
Class p^ =I -99 i-74 
F F 
Order T~ or o = 2II> 73 
K 2 
Subrang... . .-g =2.45 3-09 
The very close similarity of the two analyses is striking, all the respective figures 
being almost close enough to be satisfactory duplicates, except in the case of silica, 
which is higher by 2 per cent in II than in I. In this connection it must be said that 
the specimen of this rock differs from the other in containing a very small amount 
of glass, instead of being holocrystalline as is the rock from Fontana di Fiescoli. 
Corresponding to this chemical difference the norm of II shows not only none of 
the abundant olivine of I, but hypersthene in its stead and even a trifling excess 
of silica which appears as normative quartz. 
As regards the classificatory positions of the rocks little need be said. They 
are much alike and fall well within the boundaries of the several divisions, except 
that II is rather close to the border of salfemane and might strictly be regarded as 
transitional, that is, belonging to the magma prowersose-ciminose, though this 
refinement seems to be scarcely advisable. This difference in position is, of course, 
connected with the presence of hypersthene in the norm of II rather than olivine, 
and is a consequence of its higher silica. 
Mode. As one of the rocks analyzed is holocrystalline and the other contains 
a small amount of glass, and as their norms also differ somewhat, the modes of the 
two were determined separately. On account of the very fine-grained charac- 
ter of the groundmass and its felted texture, the mode could not be estimated in its 
entirety by Rosiwal's method, though the amounts of the phenocrysts were easily 
ascertained. 
In calculating the mode of the Fiescoli rock few readjustments were needed. The 
normative amount of olivine was taken as the modal, as this agreed with that meas- 
