THE ROMAN COMAGMATIC REGION. 
solidification the glassy cement. This view of their origin is that held by Doctor 
Cross and Professor Iddings, who very kindly examined my specimens and remarked 
on their similarity to the numerous flow-breccias of the Western States, with which 
they are very familiar. This explanation of their texture agrees with the observed 
occurrences in the field, where they appear for the most part to be flows rather than 
bedded tuffs, and yet have left unaltered the subjacent rocks, and it harmonizes the 
divergent views as to their origin. 
Chemical composition. A specimen of the breccia variant was chosen for anal- 
ysis, rather than one of the lava, as it is locally far the more important. The analysis 
calls for no special comment, except that it indicates that the rock is quite fresh. 
The magmatic position of the rock is not very close to the center point of any of the 
divisions, as is evident from the ratios given, except in the case of the subrang, where 
the alkalis are almost equal. The analysis by Ricciardi of a closely similar rock, 
though transitional between the typical peperino and that with large feldspars 
according to Mercalli, is given for comparison, and the general resemblance is rather 
close. It must be remarked, however, that the figures in Ricciardi's analyses are 
often open to doubt as to their accuracy, in many cases demonstrably widely 
inaccurate, so that the analysis is of little value. For this reason the norm was not 
calculated. 
Chemical Compositon of Sorianal Harzose [Biotite-latite]. 
I. 
II. 
I. 
II. 
SiOj 
59.41 0.990 
19.06 .187 
1.87 .012 
3.42 .470 
2.05 .051 
4.09 .073 
2.58 .042 
5.29 .056 
0.64 
58.78 
16.97 
i-i3 
2. IO 
1.46 
7-27 
3-67 
4-l8 
H 2 O- 
CO 2 
0.91 
none 
I. 00 O.OI2 
0.29 .002 
n.d 
n.d 
n.d 
3.60 
n.d 
0.32 
0.44 
trace 
0.17 
A1 2 O 3 
Fe 2 O 3 
TiO 2 
FeO 
p,O e 
MgO 
SO, 
CaO 
Cl . . . . 
Na 2 O 
MnO 
Kn 
H 2 O + 
100.61 
100.09 
I. Sorianal harzose breccia [biotite-latite]. La Cava, near Viterbo, Ciminian District. H. S. 
Washington, analyst. 
II. Peperino. Bagnaia, near Viterbo, Ciminian District. L. Ricciardi, analyst. Cf. Mer- 
calli, Mem. Ace. Line., XX, 1903, p. 12. 
Q 
Norm of I. 
ii . 76 
II . 76 
Or 
| 
Ab 
22 . OI 
. 7i . co 8> 6r 
An 
18 ?; 
C 
2 . 3^ 
2 . 3^ 
Hy. . 
8 14 
8 14 
Mt 
2 78 } 
i 
11 
I.S2 \ 
4 . 60 13 - 4^ 
Ap ... 
o. 67 
o 67 
Rest 
99.02 
I . c; 
Class. 
Ratios of I. 
Sal 
' 'Fern 
Order 
Rang... 
Subrang. 
K 2 O' + Na 2 O' 
CaO' 
'Na 2 O' 
= 6.39 
= 6.08 
= 1.48 
= i-33 
100.57 
