THE ROMAN COMAGMATIC REGION. 
NOTES EXPLAINING HEADINGS OF PRECEDING TABLE. 
I. Cumal nordmarkose-phlegrose [phonolitic trachyte]. Monte di Cuma, Phlegrean Fields. 
Washington, analyst. Cf. Am. Jour. Sci., VIII, 1899, 287. 
II. Cumal nordmarkose-phlegrose [phonolitic trachyte]. Monte Nuovo, Phlegrean Fields. 
Washington, analyst. Cf. Am. Jour Sci., VIII, 1899, 287. 
III. Cumal phlegrose [phonolitic trachyte]. Block in tuff, Via Aurelia, 4 km. SW. of Viterbo, 
Ciminian District. Washington, analyst. Cf. Jour. Geol., IV, 1896, 849. 
IV. Cumal phlegrose [phonolitic trachyte]. Monte Cimino, near Viterbo. vom Rath, analyst. 
Zeits. d. d. geol. Ges., XVIII, 1866, p. 581, alkalis interchanged? 
Norms. 
Or 42 . 26 
[. 
43 
78 1 35 
92-55 
77/ o 
o 
2 
40 o 
7.08 3 
68 o 
o 
o 
II. 
-37) 
37^78.74 
:H 88 - 95 
. 70 V JO. 21 
.OO ) 
90 
h 7-54 
. 70 ) , 
.32) I0 ' 62 
.76 [ 3.08 
.00 ) 
00 
Ab . ?o. 10 81 . 
An 2.22 
Ne . . . 7 . o s 
HI* 0.82 8 
Th* . . . . . . o . oo 
Di . . .1 .04 ^ 
Wo ... o . 46 f 2 . 
Ac .000) 
Mt 1.86 ) 
11 .. . i. 06 } 4. 
Hm i 76 ) 
Ap . . .0.00 
99 - 6 3 
Rest 0.34 
99 
o 
57 
.60 
99-97 
Class 
IOO 
Ratios 
Sal 
17 
I. II. 
= 13.07 8.38 
= 9-55 7-7i 
Order 
Fern 
F 
"L 
CaO' 
K 2 O' 
= 0.69 0.73 
"'Na 2 O' 
53-93 
26.78 
2.78 
6.96 
O. 12 
0.28 
0.65 
2.44 
O.OO 
2.32 
1 .06 
1.76 
III. 
83-49 
7-3 6 
3-9 
5-14 
91.05 
8-57 
o-34 
99.62 
i. 02 
100.64 
III. 
10.62 
n-37 
17-3 
1.23 
*For HI and Th see p. 15. 
From these norms and ratios it is seen that the rocks from the Phlegrean 
Fields approach the border of the dosodic subrang nordmarkose, like the ischial 
type, and they might be considered as transitional and the subrang be called nord- 
markose-phlegrose. On the other hand, the Vico rock is close to the center of 
the sodipotassic subrang, and the potash indeed slightly surpasses the soda. In 
other respects they are all well within the limits of class, order, and rang. 
Modes. The mode of the Cuma specimen, both as calculated from the norm 
and as measured by Rosiwal's method, is given below. In the former the amount 
of sodalite was calculated on the basis of the figures for chlorine reported, and is 
seen to be very close to the measured value. As the composition of the aegirite- 
augite was unknown, and the whole of the normative hematite would yield far too 
much aegirite, an amount of Na 2 O (.004) equivalent to that of CaO in the wol- 
