no THE ROMAN COMAGMATIC REGION. 
Norm oj I. Ratios o) I. 
Or .................. 20.85 I , >. Sal 
Class ................. v =2.14 
Ne .................. n-93 
Order 
Ap .................. 0.92 0.92 
98.59 K 2 O' 
Rest ................ i. oi Subrang .............. ^-^, =2.08 
99.60 
The analysis shows the generally central position of this type in the various 
classificatory divisions, in which it differs somewhat from the other, though the 
similarity between the two analyses is marked. 
Mode. Owing to the character of the groundmass, Rosiwal's method is not 
applicable to this type, though the relative amount by volume of the leucite micro- 
phenocrysts was determined rather satisfactorily. Recalculation of the norm was 
somewhat uncertain, because of ignorance of the relative amounts of alkali-feldspar 
and nephelite, but as the amount of the latter was small, the error involved in the 
arbitrary assumptions made as to it will not affect the result seriously. Assuming 
that it has the composition below and that about 5 per cent is present, the follow- 
ing mode was obtained, which may be regarded as closely approximating to the 
truth. The amount of leucite shown agrees reasonably well with the measured 
amount, especially when the leucite megaphenocrysts are taken into consideration, 
though it is somewhat higher. The nephelite given here exists for the most part as 
glass. 
Orthoclase, Or, Ab : ............................... 5.5 
Labradorite, Ab 2 An 3 ............................. 13.9 
Leucite ......................................... 36. 2 
Nephelite ....................................... 8.3 
Augite .......................................... 30.4 
Ores ............................................ 4.7 
Apatite ......................................... i . o 
100. o 
Occurrence. This type is especially well represented in the Sabatinian Dis- 
trict, practically all of the extensive flows of leucite-melaphyre (leucitite) south of 
Lake Bracciano belonging to it. Some prominent localities are Crocicchie, the quarry 
of L'Uomo Morto, Grotta Lobbra, La Toraccia, and other points along and close to 
the south shore, and the Osteria Nuova and elsewhere in the neighborhood of Santa 
Maria di Galera. The type also occurs in the Vulsinian District, as at Trebia- 
nello and II Giglio to the east of Lake Bolsena, and probably also in the Latian 
and Auruncan districts, though none of my specimens can be definitely referred 
to this subrang. 
Name. The type name is derived from the locality of Santa Maria di Galera, 
one of the oldest-known occurrences. 
