_Average quantitative composition of the Earth’s Crust._—The total thickness of the outer shell of the earth, thus far known to us, does not exceed about 95,000 feet, as observed in the accessible rock deposits. Estimates of the proportions in which the more abundant elements contribute to the composition of these constituent rocks, have repeatedly been made. The latest and most widely accepted of these, by F. W. Clarke, of the U. S. Geological Survey, is given herewith. It includes the constituents of the sea and atmosphere as well; these two constitute about 7 per cent of the whole, 93 per cent being solid rocks. RELATIVE ABUNDANCE OF THE ELEMENTS TO A DEPTH OF TEN KILOMETERS. SOLID CRUST OCEAN MEAN, (93 PER CENT). (7 PER CENT). INCLUDING AIR. Oxygen 47.29 85.79 49.98 Silicon 27.21 25.30 Aluminum 7.81 7.26 Iron 5.46 5.08 Calcium 3.77 0.05 3.51 Magnesium 2.68 0.14 2.50 Sodium 2.36 1.14 2.28 Potassium 2.40 0.04 2.23 Hydrogen 0.21 10.67 0.94 Titanium 0.33 0.30 Carbon 0.22 0.002 0.21 Chlorin 0.01 2.07 0.15 Phosphorus 0.10 0.09 Manganese 0.08 0.07 Sulphur 0.03 0.09 0.04 Barium 0.03 0.03 Nitrogen 0.02 Fluorin 0.02 0.02 Chromium 0.01 0.01 It will be noted that one-half of the total consists of oxygen, and that nearly 86% (or 47.29% of the 49.98%) of this amount is contained in the solid rocks; nearly 2.50% of the remainder in sea and other water; and .41% in the atmosphere, in the free condition, in which it serves for the respiration of animals and plants, and for the various processes of slow and rapid combustion, or “oxidation.” This relatively small proportion of the whole, is, nevertheless, the most directly important for the maintenance of organic life. _Oxids Constitute Earth’s Crust._—The vast predominance of oxygen in the above list suggests at once that most of the other elements must exist in combination with it, _i. e._, as “oxids.” H. S. Washington[2] has lately revised the estimates heretofore made, on the basis of a very large number of analyses made by him and others, of rocks within the United States, and gives the following table; alongside of which is placed a revised estimate by Clarke, which also includes rocks from abroad; both being given in terms of oxids of the several elements.