PETROGRAPHY. 
Chemical Composition of Orvietal Vicose [Leucite-tephrite], 
101 
I. 
I. 
SiO 2 
SO. 24 
o.8?8 
TiO 2 
I . 42 
.181 
ZrO 2 
Fe 2 O 3 
2. 54 
.016 
P 2 Oc.. 
FeO 
s.68 
.070 
SO 3 
MgO . . 
3 6 5 
.001 
s 
CaO 
7.8? 
MnO . . . 
n d 
Na 2 O 
.040 
BaO 
K 2 O 
7.4"? 
.080 
SrO 
trace 
H,O + 
CO 2 
none 
100.58 
* Former figure = 2. 97. 
I. Orvietal vicose [leucite-tephrite]. Monte Cavallo, near Orvieto, Vulsinian District. Wash- 
ington, analyst. Jour. GeoL, V, 1897, p. 370. 
Norm. 
Ratios. 
Or 
. AO 
o? 
An 
16 
96 
56.99 
Lc 
j 
40 
Ne 
ii 
14.85 
Di 
. is 
46 
Ol 
c 
1 
74 
21.20 
Mt 
71 
. 
11 
2 
08 
5-99 
Ap .. 
I 
i .0? 
3 
Rest 
IOO 
o 
06 
fir, 
71.84 
28.22 
Class. 
Sal 
'Fern 
= 2-54 
Order 
Rang 
Subrang 
Na 2 O' 
100. 71 
The analysis resembles that of the bagnoreal type, though potash is here some- 
what lower, and the general character slightly more femic. It will be noted that 
the correction of the figure for Na 2 O not only lowers the previous summation, 
which would be 101.03 with the additional determination of BaO, to a satisfactory 
figure, but also carries the magmatic position from sodipotassic borolanose (II. 6. 
2.3), where the type was formerly reckoned as belonging,* to the dopotassic subrang 
vicose (II. 6. 2. 2), and thus in accord with the numerous other similar leucitic rocks 
of the region, which are all dopotassic. 
Mode. As was the case with the groundmass of the preceding type, this rock 
did not lend itself well to measurement of the mode under the microscope, though 
a rough estimate was made to control the calculation of the mode from the norm, 
which was carried out as usual. In estimating the barkevikite this mineral was 
assumed to have the composition of the similar hornblende in the pulaskose (soda- 
lite-syenite) of Square Butte.f 
* Washington. Prof. Paper U. S. Geol. Surv. No. 14, 1903, p. 297. It was remarked here that it is "near 
subrang 2 of essexase," that is, vicose. 
t Lindgren and Melville, Am. Jour. Set., XLV, 1893, p. 292. Cf. Pirsson, Bull. U. S. Geol. Surv. No. 237, 
i cos, p. 66. 
