surface of the continents would be lowered by solvent erosion alone, to the extent of one foot in 32,833 years. In some areas the rate is much more rapid, in others it is slower; but the average is as close as can be computed with the data now in hand. Its uncertainty may be as great as ten per cent., or perhaps even greater. The chief uncertainty is due to our lack of precise knowledge concerning the greater African and Asiatic rivers.

From the ratio between fluviatile and marine sodium the age of the ocean can be calculated. The ocean contains 14,611 X 1012 metric tons of sodium, and the rivers contribute 175,040,000 tons annually. Hence, if the ocean were originally fresh, its entire content of sodium would be supplied by the rivers in 83,472,000 years. This form of calculation was first applied by Joly,12 whose work is well known; and has since been discussed by Sollas13 and also by myself in the memoir already cited. The quotient thus obtained, however, is subject to various corrections, which have been considered by the authors named above, and which operate in opposite directions. Whether they compensate or not it is impossible to say. The calculation, so far, assumes a uniform rate of supply since the surface of the earth took on its present form, and that assumption has been well criticized by Becker.14 He shows that in all probability the rate is diminishing, for the reason that the exposure of fresh rocks, of unleached material, is constantly growing less and less, and the true age of the earth since stability was established, lies between 55 and 70 millions of years. The higher of these values appears to be the more probable. If, however, the ocean were primitively saline, the quotient representing its age would be still smaller.

Sodium tends to accumulate in the ocean, while the other saline radicles added to it are more or less precipitated as solid deposits on its floor. Calcium and magnesium are removed as carbonates, silica goes to build the skeletons of radiolarians, diatoms, and so forth; potassium is taken to produce glauconite, etc. These deposits or sediments cover vast areas to an unknown thickness, but their 12 Trans. Roy. Soc. Dublin (2), Vol. 7, p. 23; Rep. British Assoc. Adv. Sci., 1900, p. 369.

annual increment can be approximately determined. If, from the yearly contributions of rivers the amount of each radicle remaining in solution is subtracted, the rate of chemical sedimentation becomes known. In order to make this calculation, the age of the ocean must be assumed; but variations in the latter estimate affect the results but little. For example, the ocean contains 571.7 X 1012 tons of dissolved calcium, which, divided by the age, gives the annual addition. If the age of the ocean is 100,000,000 years the annual increment of calcium in solution is 5,717,000 tons; if only 50,000,000 years it is 11,434,000 tons. Subtracting these quantities from the total calcium of the river waters the remainders become 551,953,000 and 546,236,000 tons respectively, the difference being much less than the uncertainties in the data employed. If, for the sake of uniformity, we take the uncorrected age of the ocean, 83,472,000 years, the chemical or biotic sediments are represented by the following annual quantities.

These are the quantities of the several substances annually removed from solution in the ocean, which, in combination assume the following form.

The last group of figures needs some explanation. From the analyses of oceanic sediments published in the reports of the Challenger Expedition I find that the ratio between sulphate calcium and carbonate calcium is 1:45.5. Calcium, therefore, is apportioned between the two salts in that ratio, but much of the SO, radicle is left

unaccounted for. Part of it goes to form pyrite, and part is decomposed by organic agencies and lost, but the proportion of loss is unknown. It is, doubtless, large. The potassium which is taken up by clays or else in glauconite is in either instance represented as silicate, and hence a part of the silica is regarded as in combination. The sesquioxides are calculated as limonite, although a part of them is certainly alumina; but no refinement of a calculation here would change the order of magnitude as given. The several orders of magnitude are probably close to the truth, and we may say with much confidence that the precipitates, including such substances as coral, shell, diatomaceous ooze and what not are formed at a rate of something like 21 X 108 metric tons a year, plus a small but undefined allowance for that part of the sulphur which has been fixed as pyrite.

At the figure given, chemical sediments are now forming in the ocean sufficient to cover 88,000,000 square miles of the sea floor to the depth of 0.0001337 inch annually. The whole area of the ocean is 139,440,000 square miles, but the portion covered by the red clay, where the precipitation is relatively insignificant, must be deducted. If the rate had been uniform throughout geological time, 83,472,000 years these sediments would form a layer about 930 feet deep, but such a calculation is unsound. Large areas of what were once marine sediments are now land, and, moreover, neither the rate nor the distribution of the deposits can have been uniform. The limestones that are forming now are largely derived from the solution of older deposits, Cambrian, Silurian, Devonian, Cretaceous, etc., and their carbonates have been deposited in the ocean, not once only, but possibly several times. In the earliest geologic eras, when sediments began to form, the proportion of carbonates to other salts thrown down must have been much smaller than today. An average thickness of 930 feet over the assumed area is therefore a great exaggeration; and needs to be reduced.

It is probably impossible to determine, with any approach to precision, the actual quantity of marine sediments that have been formed. We can, however, make a plausible estimate, which shall, at least, give us some conception of their order of magnitude. It has

already been shown that the limestones, which are mostly of marine origin, have a volume of 3.916,400 cubic miles. With a specific gravity of 2.7 their mass becomes 42,092 X 1012 metric tons. From the figures given on p. 230 ante, the calcareous and magnesian sediments are now forming at a rate bearing a certain ratio to that of the other deposits, the limonitic and siliceous residues. This ratio, which is roundly 1,650:452, if constant throughout geologic time, would give for the latter class of sediments, proportional to the limestones, a mass of 11,664 X 1012 tons; the sum of both classes of precipitates being 53,756 X 1012 tons. The corresponding average thickness over the sedimentary oceanic area would then be 287 feet, or less than one third of the figure previously given. The actual thickness, however, must be much less; for a large part of the once marine sediments are now elevated into land. According to the best estimates, the land area of the globe is now covered by 23 per cent. of archæan and eruptive rocks, and 77 per cent. of sedimentaries. Adding this sedimentary area to that of the ocean, the total becomes 132,180,000 square miles, and the average thickness of the chemical sediments reduces to 191 feet. At the crude value assigned to geologic time this represents a rate of deposition of only 0.000027 inch annually. If the age of the earth is less than 83,472,000 years, the mean annual rate of deposition will be proportionately increased, but not to anything like the present magnitude.

15

Whether the ratio assumed between the calcareous and siliceous sediments is justifiable or not, is a question admitting of argument. It seems, however, probable, that in the earliest geologic ages, when the land area was occupied principally by igneous rocks, the salinity of the rivers was relatively low, but the proportion of silica to lime in the waters was higher. This suspicion is justified by a study of the river waters of today, especially those issuing from granitoid areas. In such waters silica is often in excess of lime, while in waters from sedimentary areas the reverse is commonly true. The ratio here assumed represents a balancing between waters of both classes, and is therefore as legitimate as any other which might be chosen. Here it must be borne in mind that we are dealing with probabilities only, nothing more.

15 Von Tillo as modified by Becker. See Becker's memoir already cited.

So far, the mechanical sediments, such as silt and sand, have not been considered. From the surface of the United States, according to Dole and Stabler,16 the rivers annually carry to the sea 270,000,000 tons of dissolved substances, and 513,000,000 tons in suspension. If this ratio, which is only approximate, should hold for the whole world, the quantity deposited in the ocean during geologic time would be 102,370 X 1012 tons, and the total sedimentation, chemical and mechanical, becomes 156,126 X 1012 tons. This quantity, distributed over the entire sedimentary area, continental and oceanic, gives an average thickness of about 550 feet, or 0.000079 inch a year.

The total volume of the marine sediments thus computed, is 13,873,000 cubic miles. The volume remaining in the ocean is very nearly two thirds of this figure, 9,239,000 cubic miles. The volume of all the secondary rocks derived from the decomposition of igneous rocks was previously found to be 78,338,000 cubic miles. Hence the portion now on the land area of the globe amounts to 69,099,000 cubic miles of rock, consisting in great part of materials which were never transported very far from their original place of formation.

To the foregoing estimates of the oceanic sediments at least one large but undetermined correction needs to be applied. The ocean receives great quantities of dust, representative of aerial erosion, and also quantities of volcanic ejectamenta. For these nonfluviatile additions no valid estimates can yet be made. The major portion of them, however, must come from disintegrated sedimentary rocks, sands, and soils, and so do not affect to any serious extent our estimates of rock decomposition. The oceanic share of the sediments should be increased, but less than appears at a first glance. The marine sediments now on land must include a part of the contributions made to the ocean by atmospheric transportation. The actual distribution of the sediments is naturally very uneven. They are probably thin near the margin of the red clay, and thick along the continental shelves. Coral rock, for example, has been bored to a depth of 1,100 feet without reaching its limit. The mechanical sediments are of course mainly deposited relatively near to shore.

10

19 U. S. Geol. Surv. Water Supply Paper, No. 234, p. 83.