86 
THE ROMAN COMAGMATIC REGION. 
the rock, is of the usual very pale gray and for the most part in subhedral to anhedral 
prismoids of about the same size as the leucites. Brown biotite is present in very 
subordinate amount, not over 4 per cent, in the form of small, irregular patches 
between the subhedral minerals, not that of tabular phenocrysts. About the same 
amount of magnetite occurs in the customary small grains. A little olivine was 
found in small crystals, and there is a very little interstitial nephelite cement, the 
last product of solidification. 
Chemical composition. An analysis was made of a specimen from the massive 
flow on which the town of Toscanella is built, taken from below the town in the 
ravine of the Marta, near the west gate. The analysis does not call for much 
comment here. It will be noticed that the silica is quite low, as well as the potash, 
while the lime is rather high. 
Chemical Compositon of Orvietal Auruncose [Leucite-tephrite]. 
I. 
I. 
SiO 2 
51.21 o 
8^4 
CO 2 
none 
A1 2 O 3 
18.28 
1 70 
TiO 2 
i .43 
0.018 
Fe 2 O 3 
3.. O7 
OIO 
ZrO 2 
none 
FeO 
4- 10 
<x8 
P 2 O, . . 
O. 3? 
.OO'? 
MgO . . 
2.4.7 
087 
SO, . . 
none 
CaO 
7.86 
140 
s 
none 
Na 2 O 
2 40 
040 
MnO 
nd. 
K 2 O 
6.60 
O7O 
BaO 
o. 10 
HO4- 
H 2 O 
o 16 
99-77 
Orvietal auruncose [leucite-tephrite], Toscanella, Vulsinian District. Washington, analyst. 
Norm. 
Ratios. 
Or 
..38. 
0"" 
1 
Ab 
* 1 
17 
67 
"7" 
An 
. . IO , 
T* 
I ' 
' ] 
72 . 66 
Ne 
6 
. 3O 
6 
7O 
Di 
. . i?, 
56 1 
i 
Ol 
;i7- 
14 i 
I 
Mt 
. 4. 
41 
1 
-25-31 
11 
2 , 
T I 
8 
17 - 
1 
Ap .. 
I , 
1 
. O2 
) 
Rest , 
9 8, 
O. 
97 
8<* 
Class 
Order. 
Sal 
Fern 
F 
'L 
2.91 
Rang . . 
Subrang 
' + Na 2 O' 
CaO' 
K 2 O' 
- Na 2 O' 
= 1.24 
i-75 
99-79 
Mode. The mode of the Toscanella rock was determined both by Rosiwal's 
method and by calculation from the norm. In the former the amounts of leucite 
and biotite could be measured with a very considerable degree of exactness, while 
the two feldspars and the nephelite were necessarily measured together, as was the 
case with the augite and olivine. In calculating the mode from the norm, therefore, 
the measured amounts of leucite and of biotite were assumed to be correct, as some 
such assumptions were necessary to distribute the potash between orthoclase, leucite, 
and biotite, and the normative olivine molecules between modal biotite and olivine. 
