PETROGRAPHY. 39 
In this way the following calculated mode was obtained: 
Orthoclase, Or s Ab 2 47 . 85 
Labradorite, AbiAn 2 19-91 
Leucite 17.22 
Augite 8.21 
Ores 5 24 
Apatite 0.40 
98.83 
H 2 O, etc 0.99 
99.82 
It is clear that the discrepancy between the observed and the calculated amounts 
of leucite, 36.8 and 17.2 per cent, can not be explained by the corrections needed 
for inclusions, and it has been shown above that the larger amount of leucite as 
revealed by Rosiwal's method must be close to the truth. As, furthermore, by no 
possible means can more leucite be calculated from the magma without setting 
silica free, and there is no reason to believe that the analysis is seriously incorrect, 
it follows that the only allowable explanation of the discrepancy is that the sample 
analyzed did not correspond to the rock as a whole, but contained an unduly large 
amount of groundmass and an unduly small amount of the leucite phenocrysts.* 
That this explanation is the correct one is the more probable when it is con- 
sidered that the analysis was made by me some eight years ago, when the 
importance of crushing up a sufficiently large piece of rock had not impressed itself 
upon me, and when the coarsely porphyritic character of the rock and of the brittle- 
ness of the leucite phenocrysts are taken into account. If a rather small piece was 
taken, it would be probable that the sample analyzed would contain relatively more 
groundmass than the rock itself. This conclusion is borne out by the fact that the 
modes of the San Rocco specimen show considerably more of the groundmass con- 
stituents than in the other specimen. Attention is called to this otherwise rather 
unimportant point, as it serves to emphasize a feature of the analysis of rocks the 
proper preparation of the sample on which I have laid stress elsewhere.f In view 
of the conclusions above, the analysis (II) of the San Rocco vulsinose can not be 
considered as representative of the rock magma, and must be disregarded in future 
discussions. 
Occurrence. This type has been found with certainty only in the Ciminian 
District, where it is very abundant, many of the lava flows of the Vico Volcano 
belonging here. Among localities where especially good specimens were collected 
may be mentioned several flows in the east inner wall of Monte Vico below San 
Rocco, flows near San Martino, northwest of Lake Vico, especially above the 
church and below the town at the Contrada di Merlano, and southwest of Lake 
Vico, as at the Sorgente di Grignano and the Ponte delle Cese. 
* As the silica percentage of leucite is 55.05, and therefore close to that of the rock, the removal of leucite would 
not materially affect the silica shown by the analysis. 
tH. S. Washington, P. P. 14, U. S. G. S., p. 18, 1903; and The Chemical Analysis of Rocks, New York, 
1904, p . 47- 
