COMPOSITION OF ROCKS. vi 
Ferrous oxide.—1,093 determinations. Average, 3.40 per cent. In 38 analyses the 
oxides of iron were not separately determined, and in them the figure for iron is 
excluded from the calculation. 
Magnesia.—1,163 determinations. Average, 3.96 per cent. 
Lime.—1,384 determinations. Average, 4.89 per cent. 
Soda.—1,442 determinations. Average, 3.47 per cent. 
Potash.—1,434 determinations. Average, 3.00 per cent. 
Water.-—In 288 analyses, total water was determined; average, 1.86 per cent. In 752 
analyses, water lost below 100°-110° was estimated; average, 0.48 per cent. In 
816 analyses, water retained above 100°-110° gave an average of 1.51 per cent. 
Combining these data, the probable percentages are, H,O—, 0.47, and H,O+. 
1.48 per cent. 
Titanic oxide.—989 determinations. Average, 0.74 per cent. 
Zirconia.—281 determinations. Average, 0.026 percent. These determinations were 
not made so generally as to give the average any great importance. 
Phosphoric oxide.—995 determinations. Average, 0.26 per cent. 
Baryta.—710 determinations. Average, 0.11 per cent. 
Strontia.—596 determinations. Average, 0.04 per cent. 
Manganese oxide.—1,011 determinations. Average, 0.10 per cent. 
Carbon dioxide.—599 determinations. Average, 0.50 per cent. This is doubtless too 
high, for in many rocks carbon dioxide was not looked for. In 142 of the analyses 
in which it was reported the percentage was zero. 
Lithia.—Commonly reported in traces, but often absent. Reckoning a ‘‘trace” as 
0.01, the average of 571 analyses is 0.011 per cent. This is probably not far from 
a true estimate. 
Nickel oxide.—273 determinations. Average, 0.026 per cent, which is probably too 
high. 
Chromic oxide.—262 determinations. Average, 0.05 per cent. Probably too high. 
Vanadium trioxide.—Only 83 determinations are reported; in average, 0.024 per cent. 
Determinations too few to give this value any weight. The figure, however, is 
not without interest. : 
Chlorine.—Determined or proved to be absent in 258 cases. Average, 0.065 per cent. 
Probably too high. 
Fluorine.—Only determined or proved to be absent in 90 analyses. Average, 0.11 
per cent, which is undoubtedly too high. An inferior limit, however, may be 
fixed with reference to P,O,;. If we assume that to represent apatite, the equiva- 
lent amount of fluorine should be 0.023 per cent. The true average must be 
slightly higher. 
Sulphur.—This element is reported in three forms—as 8, as SO;, and as Fe&,. 
Reported as S, 253 analyses give an average 0.051 per cent. As SO;, 321 analyses 
give 0.11 per cent, equivalent to 0.044 per cent of S. In 143 analyses 0.68 per 
cent FeS, was found, equivalent to 0.36 of sulphur and 0.32 of Fe. The latter 
must be taken into account in estimating total iron. General average for sulphur, 
0.109 per cent. 
a See Hillebrand, W. F., Bull. U. S. Geol. Survey No. 167, 1900, on the vanadium found in 57 rocks. 
