THE FORMATION OF LEUCITE. 
relations between silica and potash on the one hand, and between potash and soda 
on the other, are largely the determining factors in the rocks of the region, the con- 
ditions of solidification being similar. The data on which to study this question 
are given in the annexed table, in which the amounts of silica and potash are given 
both in percentages and as molecular ratios, together with the ratios (molecular) 
* and . T 2 -^ . The rock numbers are the same as in the table of analyses 
K 2 O Na 2 O 
on p. 146, and the leucitic rocks are indicated by an asterisk (*), those in which the 
mineral is merely sporadic and accessory, as the cumal phlegrose and arsal mon- 
zonose, being considered non-leucitic. 
Relations of Silica, Potash, and Soda in the Roman Rocks. 
SiO, 
K,O 
SiO, 
K,O 
K,O 
Na,O 
K,0 
Na,0 
I 
6l.88 
6. 72 
U. C.2 
0.64 
23 . . 
c6. 71: 
c, .02 
i=;.o2 
0.83 
2 
CQ. 70 
7. IO 
13.12 
o. 60 
24 
C4. 72 
6.87 
12 -49 
1.30 
2 . . 
60. 33 
7- 3O 
12.90 
O. 73 
2?* .. 
^2. 37 
7.47 
IO.OI 
i . 70 
4 . . 
en. 24 
9. 14 
IO. 17 
I . 23 
26* 
co. 86 
7. ic 
II .01 
2. 2O 
c . . 
61 . 62 
7.6o 
12.68 
0.87 
27* . . 
^1 . 21 
6.60 
12. 2O 
I- 75 
6 
=;8.o8 
8.87 
2 . O7 
1 .07 
2 8 
cc .60 
4.41 
IQ . 74 
1 .00 
7 ... . 
c,7. c.8 
8.68 
IO. 12 
1.86 
20* . . 
54.8^ 
10.40 
8.23 
2. 36 
8 . ' 
c.7. co 
8. 30 
10.78 
I. 74 
3O* . . 
<I . 2O 
10.63 
7. cc 
3. 32 
o* . 
?6 IQ 
TO. 47 
8.36 
2 .43 
31* .. 
<;o. 68 
9.38 
8.45 
2.38 
10* . . 
ec 17 
n.c,8 
9. 02 
2 . 76 
32* . . 
co. 36 
o. 30 
8.39 
3.13 
ii 
C7 60 
8.71 
10. 02 
1.63 
33* .. 
co. 24 
7.4C 
10.48 
2.OO 
12* 
cc .07 
8.6c, 
0.87 
I .43 
34* 
47. 6q 
7.47 
9.90 
1.78 
13* . . 
cc.87 
IO.4Q 
8.31 
1. 4? 
3C* 
47.89 
8.23 
9.18 
2.08 
14* 
c.o.2i; 
II . 32 
6.98 
1.43 
36* 
47. 20 
7.63 
9.72 
2.25 
ic. . , 
C.Q.4I 
5 . 2O 
17.68 
1 .33 
37* 
48. 10 
7.90 
9.55 
I.9I 
16 
C7. -22 
0- I s , 
9.84 
1.87 
38* 
47- 71 
7.64 
9.81 
I. 80 
17 
cc.46 
6.6? 
13. OI 
2-4^ 
39* 
47-39 
6.93 
10.82 
3-4 
18 
c.7. ?i 
6.38 
10.04 
3.OQ 
40* 
44.89 
3.63 
19.02 
2.29 
10* . . 
cc.Sc, 
8.77 
Q.QO 
I. 71 
41* 
46. 24 
6.37 
11.33 
2.52 
20* 
CC . 21 
8.4^ 
IO. 22 
1. 80 
42* 
46. 27 
8.58 
8.47 
2.07 
21* . . 
c.2. 14 
7. 24 
II . 2Q 
2.48 
43* 
47-oe. 
7.52 
9.80 
3.07 
22 
C.C, .22 
7.58 
II. 36 
I-4S 
44* 
45.99 
8.97 
8.00 
2.74 
Regarded thus in tabular form these figures seem to point to no very definite 
C * /"\ 
kw. While the non-leucitic rocks show generally high = ^ ratios, we find 
Jv 2 U 
almost equally high figures for this (19.74 and 19.02) in the non-leucitic and 
K O 
the leucitic types. Similarly the other ratio, 2 , does not show any very marked 
JN cLavJ 
regularity. But if the figures are plotted as in the subjoined diagram, the abscissae 
representing the percentage of silica and the ordinates the ratios in question, some 
interesting relations are made manifest. The dots represent non-leucitic rocks 
and the dots in circles leucitic ones. 
