THE DISTRIBUTION OF BARIUM. 191 
Whatever be the true facts it is clear that such problems are of considerable 
importance, not only to petrology but to other sciences as well, and that for their 
solution complete chemical analyses of igneous rocks and the determination of 
constituents which are usually disregarded on account of their rarity are absolutely 
essential. 
As to the minerals into which the BaO enters in igneous rocks, the data are as 
yet unsatisfactory, since many mineral analyses have suffered, like those of rocks, 
from neglect to determine the rarer constituents. As shown by the analyses in 
Hintze's Mineralogie, by those in Bulletin No. 220 of the U. S. Geological Survey 
(1903), and as pointed out by Vogt in the paper cited above, BaO has been found 
in many feldspars, though it has been looked for only exceptionally. It would seem 
to be most common in orthoclase, soda-orthoclase, and microcline, presumably as 
the hyalophane molecule, and has also been noted in a few soda-lime feldspars, 
where it doubtless exists in the molecule of celsian. Its presence has been detected 
in both muscovite and biotite, sometimes in very considerable amount, up to as high 
as 6.84 of BaO, while it seems to be absent from, or present only as traces in, the 
pyroxenes and amphiboles. It will be noted that these facts of distribution among 
minerals, showing that BaO is most prominent in the potassic minerals and least 
so in those poor in potash, are in line with the observations as to the distribution 
of BaO among igneous rocks. 
There does not seem to be any record of the presence of BaO in leucite, at 
least none has been found in a search of the literature. A large leucite from the 
viterbal vicose of Garofali in the Auruncan District was examined, and furnished 
0.09 per cent of BaO, while the similar rock from Monte San Antonio gave 0.33 per 
cent. It is expected to examine other leucites in this regard, but it would seem 
from this result that the BaO in the Italian rocks exists, for the most part, in the 
feldspars, and only subordinately in the leucite. 
