7 6 
THE ROMAN COMAGMATIC REGION. 
Norms. 
I. II. 
Or 45 4 ) 35 3 ) 
Ab 14.93^72.20) 32-49 h 77-53, 
An 12.23) SSo.oi 10.01 ) [-82.07 
Ne 7.81 7-8i) 4-54 4-; 
Di n-37 I Id 22 \ 9-i2 ) ' 
01 2 .8 5 p 4 - 22 1-67 r- 
Mt 3-02J 6 19-42 3-25 s . 68 17-28 
H 1-67$ y l 2.43 \ - 
Ap 0.51 0.517 0.81 0.81 
99-43 99-35 
Rest i . 03 o . 29 
100.46 99-64 
I. II. 
Sal 
Fe^ =4 ' 12 4 ' 75 
Order I =9 ' 24 I7 ' 8 
Rang .. K2 c ^ a - =3-n 3-92 
K 2 0' 
Subrang =i-45 - 8 3 
JNa a O 
40-59 
21.48 
17-51 
4.26 
5.88 
3-32 
3-48 
1.22 
97-74 
2-13 
99.87 
III. 
6.03 
18.68 
2.05 
1.30 
III. 
79-58 
4.26 
9. 20 
4.70 
83.84 
13.90 
These analyses do not call for special comment. They are much alike in all 
respects and the rocks evidently fall well within the borders of the various divisions, 
except that that from Poggio Cavaliere lies somewhat close to the border of the 
dopotassic subrang, though not close enough to be considered transitional. The 
older analysis of the Arso rock by Fuchs, and a still earlier one by Abich, 
resemble that in II, except in their somewhat higher silica and potash and lower 
soda, but their dates and the conditions obtaining in analytical work and methods 
at the time render them not altogether reliable. 
Mode. The modes of the two specimens analyzed by me were determined and 
are here described. That of the bolsenal type can only be calculated from the 
norm, as the description of De Lorenzo is not quantitative. 
In the case of Poggio Cavaliere rock determination of the mode by optical 
methods was rendered difficult by the size and arrangement of the groundmass 
feldspars and by the presence of the nephelite cement. Nor could the relative amounts 
of the phenocrysts of orthoclase and plagioclase be exactly estimated in the absence 
of sufficiently large sections. On the other hand, the amount of augite, both in 
phenocrysts and in the groundmass, and of the biotite phenocrysts, could be closely 
judged, the augite being about 16 per cent and the biotite about 1.8 of the rock by 
weight. 
In calculating the mode from the norm the amount of biotite as thus deter- 
mined was assumed to be correct, and the proper amounts of olivine and leucite 
molecules were assigned to it. The small amount of olivine remaining, changed to 
