PETROGRAPHY. 
NOTES EXPLANATORY OF PRECEDING TABLE. 
I. Monfinal shoshonose [biotite-vulsinite]. Monte Santa Croce, Auruncan District. Wash- 
ington, analyst. Jour. Geol., V, 1897, p. 252. 
II. Radicofanal shoshonose [andesite]. Radicofani, Tuscany. Washington, analyst. Am. Jour. 
Set., IX, 1900, p. 52. 
Norm of I. 
Q ................... 4.26 4.26 } 
Or ................... 26.13) f 77-54 
Ab .................. 24. 63 73. 28) 
An .................. 22.52 
Di ................... 22.52 
Hy .................. 5.43 
Class 
Ratios of I. 
Sal 
Ap.. 
Rest. 
7.78 
0.36 0.36 
Order . 
Fern 
F 
Q 
.... 
0.17 
99.80 
Norm of II. 
Or 
20. 02 
Ab 
17. 2O 
An 
2^.8? 
Di 
10 87 
Hv.. 
. 21 27 
Ol 
I .2? 
Mt.. 
I 1O 
11 
Rest 
99-94 
O. 1C 
33-39 
3-39 
63.16 
36.78 
Rang . . 
Subrang . 
Class. . 
CaO' 
K 2 O' 
Na 2 0' 
Ratios of II. 
Sal 
' Fern 
Order 
Rang. 
F 
Q,L 
CaO' 
Subrang 
100.09 
K-.O' 
'Na a O' 
' 3-Si 
'I7.20 
' 1.16 
' 1. 00 
=1.71 
= oo 
=0.74 
= 1.09 
From the norm of I it is clear that the Santa Croce rock is well within the bor- 
ders of all the magmatic divisions, or, in other words, is an almost central shoshonose. 
The Radicofani rock, on the other hand, which carries augite and very little olivine 
as alferric minerals, but no biotite, is evidently richer in these, as it is almost in the 
salfemane class. It will be noted also that the monfinal type shows an excess of 
silica, appearing as normative quartz, while the radicofanal has deficient silica, 
giving rise to some normative olivine. 
Mode. The mode of this type could not be determined by Rosiwal's method, 
on account of the very fine grain and the mineralogical character of the groundmass, 
though the relative amounts of the phenocrysts, including all of the biotite, were 
estimated with accuracy, the results of which appear in the formal description. 
In calculating the mode from the norm by readjustment the observed amount of 
biotite was taken as a starting-point. That this is probably correct is shown by the 
fact that this amount takes up all of the normative hypersthene, with silica lowered 
to form olivine (3.84 per cent), which with an almost equal amount of leucite (3.5 
per cent), assumed to form biotite, makes just 7.3 per cent of this mineral. The 
