PETROGRAPHY. 105 
The analysis (I) resembles closely many of the earlier analyses of Vesuvian lavas. 
Compared with the analyses given in the preceding pages it is marked by its low 
silica content, which is 3 per cent less than in the types of vicose. Concomitantly 
with this decrease in silica there is an increase in the amount of lime, though the 
other constituents are not notably affected. The analysis of the later flow 
resembles the former in a general way, though potash is decidedly higher. The 
higher alumina is to be attributed in part to the non-determination of TiO 2 , 
which would be included in this, and also possibly in part to incomplete separation 
of the magnesia, as the amount of this last is rather low. 
Mode. In case of this type, as in some of the preceding, the character of the 
groundmass did not permit of satisfactory measurement by Rosiwal's method, the 
relative amount of leucite phenocrysts being the only quantitative relation which 
could be determined with precision. The mode was calculated from the norm 
on the assumptions that no orthoclase was present and that all the potash belongs 
to leucite, that the normative olivine corresponds with the modal, that the augite 
has the composition of that of Ticchiena (p. 134), and that the plagioclase has the 
composition Ab 2 An 3 . The results of this calculation are: 
Labradorite, Ab 2 An 3 *7 4 
Leucite 34-9 
Nephelite 9.0 
Augite 30. 3 
Olivine 5.6 
Ores 1.5 
Apatite 1.3 
The amount of leucite shown corresponds well with that determined optically, 
namely, 40 per cent by volume, and in general the mode above agrees with the 
appearance of the thin section. Only the amount of nephelite seems to be high. The 
figure for this was that obtained by the usual equations involving the total soda and 
the silica available after formation of leucite, augite, olivine, and the anorthite of 
plagioclase. These give an amount of albite molecule exactly right to satisfy the 
composition of the plagioclase determined optically, that is, Ab =0.013, An =0.038. 
Any notable change in the figure for nephelite will involve very serious modification 
of the plagioclase composition, rendering it an andesine, which can not be considered 
in view of the optical determinations, and will also concomitantly lead to increase 
in the amount of olivine and decrease in that of augite, which are also inadmissible. 
The amount of nephelite is readily underestimated by the eye in such groundmasses, 
and a test of the rock powder with very dilute nitric acid, according to the method 
suggested by Pirsson,* gave abundant gelatinous silica, thus proving its presence 
in considerable amount. 
Occurrence. This type is abundant in the Campanian District, many of the 
flows of Vesuvius belonging to it. Among those which I have examined may be 
mentioned those of 1822 (above Cerasiello) and 1872 (below the Observatory), 
* L. V. Pirsson, Am. Jour. Sci., II, 1896, p. 142. 
