82 SOME PRINCIPLES AND METHODS OF ROCK ANALYSIS, [bull. 176. 
have resulted so well as they are shown to have done. It should be 
mentioned that for the gravimetric tests but one or two grams at most 
were used, which accounts for the reported absence of chromium in 
two instances, this report being based on the lack of color in the aque- 
ous extract of the alkali fusion after removal of manganese. 
XVIII. VANADIUM (CHROMIUM) AND MOLYBDENUM. 
DISTRIBUTION OF VANADIUM AND MOLYBDENUM. 
The wide distribution of vanadium thoughout the earth's crust has 
in recent years been clearly established (see ante, p. 18), not only in ores 
and in coals, but in clays, limestones, sandstones, and igneous rocks. 1 
The writer has shown (loc. cit.) that vanadium occurs in quite appre- 
ciable amounts in the more basic igneous and metamorphic rocks 
up to 0.08 per cent or more of V 2 3 , but that it seems to be absent, or 
nearly so, from the highly siliceous ones. Some of their ferric alumi- 
nous silicate constituents carry still higher percentages — up to 0.13 
per cent V 2 3 in a biotite separated from a pyroxenic gneiss. Molyb- 
denum, on the other hand, appears to be confined in quantities suscep- 
tible of detection to the more siliceous rocks, and, except perhaps in 
rare instances, is not present in them in quantitatively determinable 
amount when operating on 5 grams of material. Hence the quanti- 
tative search for vanadium will usually be limited to rocks with less 
than 00 per cent of silica. The search for it even then will perhaps not 
often warrant the necessary expenditure of time, but in this connec- 
tion it is to be remembered that neglect to estimate it introduces an 
error in the figures for both ferrous and ferric oxides, which in ex- 
treme cases met with may be of considerable moment. (See p. 58, and 
also pp. 95-96.) 
DESCRIPTION OF METHOD. 
In the following method there is nothing absolutely novel except 
that chromium and vanadium, when together, need not be separated, 
but are determined, the former colorimetrically, as already described 
(p. 80), the latter volumetrically, in the same solution. 2 
Five grams of the rock are thoroughly fused over the blast with 
20 of sodium carbonate and 3 of sodium nitrate. After extracting 
with water and reducing manganese with alcohol it is probably quite 
unnecessary, if the fusion has been thorough, to remelt the residue 
1 W. F. Hillebrand, Distribution and quantitative occurrence of vanadium and molybdenum in rock 
of the United States: Am. Jour. Sci., 4th series, Vol. VI, p. 209, 1898; Chemical News, Vol. LXXVIII 
p. 216, 1898; Bull. U. S. Geol. Survey No. 167, p. 49. 
2 W. F. Hillebrand: Jour. Am. Chem. Soc, Vol. XX, p. 461, 1898; Chemical News, Vol. LXXVIII 
p. 295, 1898; Bull. U. S. Geol. Surv., No. 167, p. 44. 
