370 HENRY S. WASHINGTON 
tion that potash, and those headed “‘b” on the assumption that soda, 
has the greater affinity for silica. The amount of leucite actually 
present in the rock is given in the legend below. 
I II Til IV 
a b a b a b a b 
Ora 1521. 20))!| 327052 | SOO 20/00) Or 72) |ieLOn 5 Onli t2oulk none 
AID Ne A seretetrasoke ists E7220 ||) 25.52 )|er5e 20.) 205205|) NOS 23249025 none ms |mTesOO 
ATs 8.62 Se O2i ere. 32.3 il 2k 233 9-45 OAS pes 07 etsnO7 
TE Cresee scene ates none | 19.40 | none | 18.31 none ||| dor. |=262005|3'5)..32 
Ne.. 6.2 none S94 |) none || 12.00) |= nones || 12)578 6.84 
DD ices Oi vereventate eons 6.05 6.05 ek sy Fit] 7.98 MoO || Ages |; Aetatis 
Olle. I .05 I.05 1.87 TOV Ib 0)7/ TeOV7 6.05 6.05 
Mt.. 4-41 | 4-40 | -3-94 | 3:04 |) 2.32) 2.32) 93:48) 93.48 
cig aisetercyoinceters I.06 17.0.0 1 Oy) Taf Hate) 1.52 27,0) 1) O70 
Hm.. ORSO I GO SSON IM rexctsse. willl Ne ete cess | vstestet east e4| earatene us teton (pairs oe Ral (te eee 
Ap... 0.94) | 0.94 | ©.53,| 0253 | 0-34) |) On34.1 5 1277 1 Lams 
I. Bagnoreal ciminose (leucite-trachyte). Bagnorea, Vulsinian District 
(Rom. Com. Reg., p. 68). Modal leucite=8.8 per cent. 
II. Bagnoreal ciminose (leucite-trachyte). Monte Venere, Ciminian Dis- 
trict (Rom. Com. Reg., p. 69). Modal leucite=16.9 per cent. 
III. Foglianal vicose (leucite-tephrite). Monte Fogliano, Ciminian District 
(Rom. Com. Reg., p. 93). Modal leucite = 40.6 per cent. 
IV. Scalal braccianose (leucite-tephrite). La Scala, Mount Vesuvius (Rom. 
Com. Reg., p. 120). Modal leucite=35.6 per cent. 
On comparison of the norms and modes it will be seen that in I 
only about half the maximum possible amount of leucite has developed, 
in II the maximum is nearly reached, and in III and IV the whole 
amount of the potash possible has gone into leucite, no orthoclase 
being present in the mode of the last rock." 
It is evident that the problem is fundamentally different from that 
presented by the former sort of exceptions, where the absence of 
leucite could be readily and reasonably explained either by incomplete 
crystallization or by the mass action of the abundant and complex 
alferric minerals and nephelite. Here, on the contrary, the majority 
of these abnormatively leucitic rocks contain relatively large amounts 
t A similar comparison has been undertaken by Lacroix (Comptes rendues, Vol. 
CXLI [1905], p. 1190), in discussing the granular leucitic sommaites of Monte Somma, 
though his norms are not calculated in the regular way, as he makes all the soda enter 
albite and does not assume the normative diopside molecule with CaSiO;=(Mg, Fe) 
SiO3. 
