average value we can allow for saline masses inclosed within the solid crust of the earth, and which would not otherwise appear in the final estimates. Combining this datum with Dittmar's figures for the average composition of the oceanic salts, we get the second of the subjoined columns. Other elements contained in sea water, but only in minute traces, need not be considered here. No one of them could reach 0.001 per cent.

It is worth while at this point to consider how large a mass of matter these oceanic salts represent. The average salinity of the ocean is not far from 3.5 per cent; its mean density is 1.027, and its volume is 302,000,000 cubic miles. The specific gravity of the salts, as nearly as can be computed, is 2.25. From these data it can be shown that the volume of the saline matter in the ocean is a little more than 4,800,000 cubic miles, or enough to cover the entire surface of the United States, excluding Alaska, 1.6 miles deep. In the face of these figures, the beds of rock salt at Stassfurt and elsewhere, which seem so enormous at close range, become absolutely trivial. The allowance made for them by using the maximum salinity of the ocean instead of the average is more than sufficient, for it gives them a total volume of 325,000 cubic miles. That is, the data used for computing the average composition of the ocean and its average significance as a part of all terrestrial matter are maxima, and therefore tend to compensate for the omission of factors which could not well be estimated directly.

The average composition of the lithosphere is very nearly that of the igneous rocks alone. The sedimentary rocks represent altered igneous material, from which salts have been leached into the ocean, and to which oxygen, water, and carbon dioxide have been added from the atmosphere. For these changes corrections can be applied, and their magnitude and effect, as will be shown later, is surprisingly small. The thin film of organic matter upon the surface of the earth

According to J. Joly (Sci. Trans. Roy. Soc. Dublin, 2d ser.. vol. 7, p. 30, 1899), the sodium chloride in the ocean would cover the entire globe 112 feet deep.

can be neglected altogether. In comparison with the 10-mile thickness of rock below it, its quantity is too small to be considered. Even beds of coal are negligible, for their volume also is relatively insignificant. Practically, we have to consider at first only 10 miles of igneous rock, which, when large enough areas are studied, averages much alike in composition all over the globe. This point was established in an earlier memoir, when groups of analyses, representing rocks from different regions, were compared. The essential uniformity of the averages was unmistakable, and it has been still further emphasized in later computations by others as well as by myself. The following averages are now available for comparison:

A. My original average of 880 analyses, of which 207 were made in the laboratory of the United States Geological Survey and 673 were collected from other sources. Many of these analyses were incomplete.

B. The average of 680 analyses from the records of the Survey laboratories, plus some hundreds of determinations of silica, lime, and alkalies.

data up to January 1, 1897.

The Survey

C. The average of 830 analyses from the Survey records, plus some partial determinations. The Survey data up to January 1, 1900.

D. An average of all the analyses, partial or complete, made up to January 1, 1904, in the laboratories of the Survey.b

E. An average, computed by A. Harker, of 397 analyses of igneous rocks from British localities. Many of these analyses were incomplete, especially with respect to phosphorus and titanium.

F. An average of 1,811 analyses, from Washington's tables.

Calculated by

II. S. Washington. The data represent material from all parts of the world.

Now, omitting minor constituents, which rarely appear except in the more modern analyses, these averages may be tabulated together, although they are not absolutely comparable. The comparison assumes the following form:

Bull. Phil. Soc. Washington, vol. 11, p. 131, 1889. Also in Bull. U. S. Geol. Survey No. 78, p. 34, 1891.

See Bull. U. S. Geol. Survey No. 228, p. 17, 1904, for details.

Geol. Mag., 1899, p. 220.

Prof. Paper U. S. Geol. Survey No. 14, p. 106, 1903. In this average and in Harker's there are data for manganese, which I now leave temporarily out of account.

Although these six columns are not very divergent, there are differences between them which may be more apparent than real. Differences of summation are partly due to the omission of minor constituents; but the largest variations are attributable to the water. In two columns hygroscopic water is omitted; in two it is not distinguished from combined water; in two a discrimination is made. By rejecting the figures for water and recalculating to 100 per cent, the averages become more nearly alike, as follows:

Recalculated average composition of igneous rocks.

Of these averages, only D and F need be considered any further, for they include the largest masses of trustworthy data. A was only a preliminary computation, B and C are included under D; Harker's average contains too many imperfect analyses. D and F, however, are not strictly equivalent. Washington's average relates only to analyses which were nominally complete and made in many laboratories by very diverse methods. My average represents the homogeneous work of one laboratory, and includes, moreover, many partial determinations. For the simpler salic rocks determinations of silica, lime, and alkalies are generally all that is needed for petrographic purposes. The femic rocks are mineralogically more complex, and for them full analyses are necessary. The partial analyses, therefore, represent chiefly salic rocks, and their inclusion in the average tends to raise the percentage of silica and to lower the proportions of other elements. The salic rocks, however, are more abundant than those of the other class, and so the higher figure for silica seems more probable. This conclusion is in line with a criticism by E. P. Mennell, who thinks that the femic rocks received excessive weight in my earlier averages. Mennell has studied the rocks of southern Africa, where granitic types are predominant, and he believes that the true average should approximate the composition of a granite. A wider

Geol. Mag., 5th ser., vol. 1, p. 263, 1904. For other discussions of the data given in my former papers, see L. De Launay, Revue gén. sci., April 30, 1904; and C. Ochsenius, Zeitschr. prakt. Geol., May, 1898. Compare also R. A. Daly, Bull. U. S. Geol. Survey No. 209, p. 110, 1903, who argues that the universal or fundamental magma is approximately basaltic.

range of observation would probably modify that opinion, which, however, is entitled to some weight.

So far, my final average has only been partly given; the minor constituents of the rocks remain to be taken into account. In the laboratory of the Geological Survey the analyses of igneous rocks have been unusually elaborate, and many things have been determined that are too often ignored. The complete average is given in the next table, with the number of determinations to which each figure corresponds. In the elementary column hygroscopic water does not appear, but an allowance is made for a small amount of iron which was reported in the analyses as FeS2. When a "trace" of anything is recorded, it is arbitrarily reckoned as 0.01 per cent, and when a substance is known to be absent from a rock, by actual determination of the fact, it is assigned zero value in making up the averages.

Complete avereage of analyses of igneous rocks made in the laboratories of the United States Geological Survey.

In this computation the figures for C, Zr, Cl, F, Ni, Cr, and V are only very rough approximations. They show, however, that these elements exist in igneous rocks in determinable quantities. The elements not included in the calculation represent minor corrections, to be applied whenever the necessity for doing so may arise. For estimates of their probable amounts, the papers by J. H. L. Vogt and J. F. Kemp can be consulted. It is probable that no one of

a

"Zeitschr. prakt. Geol., 1898, pp. 225, 314, 377, 413; 1899, pp. 10, 274. See also a curious paper

Science, January 5, 1906; Econ. Geol., vol. 1. p. 207, 1905.

by W. Ackroyd, in Chem. News, vol. 86, p. 187, 1902. W. N. Hartley and H. Ramage (Jour. Chem. Soc., vol. 71, p. 533, 1897), have shown that some of the rarest elements, such as gallium and indium, are widely diffused in rocks and minerals.

them, except possibly copper, would reach 0.01 per cent. The elements not mentioned in the table can not amount to more than 0.5 per cent altogether, and even that small figure is likely to be an overestimate.

Before we can finally determine the composition of the lithosphere, the sedimentary rocks are to be taken into account; and to do this we must ascertain their relative quantity. First, however, we may consider their composition, which has been determined by means of composite analyses. That is, instead of averaging analyses, average mixtures of many rocks were prepared," and these were analyzed once for all. The results appear in the next table.

Composite analyses of sedimentary rocks.

A. Composite analysis of 78 shales; or, more strictly, the average of two smaller composites, properly weighted.

In attempting to compare these analyses with the average composition of the igneous rocks, we must remember that they do not represent definite substances, but mixtures shading into one another. The average limestone contains some clay and sand; the average shale contains some calcium carbonate. Furthermore, they do not cover all the products derived from the decomposition of the primitive rock, for the great masses of sediments on the bottom of the ocean are left out of account. The analyses of the latter are too few to give conclusive averages, but the data published in the Challenger reports"

a These mixtures were prepared by G. W. Stose. under the direction of G. K. Gilbert. The analyses were made by H. N. Stokes in the laboratory of the United States Geological Survey. See Bull. U. S. Geol. Survey No. 228, p. 20, 1904.

Volume on Deep-sea deposits, p. 198.