The It is well known that about three-fourths of the earth's surface is covered by oceans and seas. Precise measures of the relative areas of land and sea are difficult to obtain, and the present state of knowledge .
does not justify the use of more than three sig nificant figures in defining these areas. The values given below are derived from H. Wag ner's der Geographic) They do not differ widely from the values given recently by other German authorities, though they make the total ocean area about 1 per cent greater than the earlier estimates of the distinguished British geographer, Sir John Murray. The table below gives the continental areas along with their average heights and the total oceanic area along with its average depth : The total area of the continents is, therefore, 26.3 per cent of the total surface of the earth; leaving, with the same degree of precision, for the total area of the sea surface 73.7 per cent of the surface of the earth.
The average depth of the oceans according to Wagner is 3,500 metres or 11,480 feet, or, in round numbers, 2.2 miles. • The data thus given show that the volume of the oceans is 315,000,000 cubic miles, or in round numbers 1-800th of the entire volume of the earth. Assuming the density of sea water to be 1.03 times that of pure water, and the den sity of the latter to be 62.3 pounds per cubic foot, the mass in the oceans is found to be 1.49X10" tons of 2,000 pounds, or one and one-half million million million tons.
It is interesting to contrast this mass of the oceans with the mass of the continents which lies above the sea-level. The average heights of the continents given in the above table require an average height for the entire land area of about 2,300 feet. This combined with the area of the continents gives for the volume of the continents above sea-level 22,500,000 cubic miles. If to this mass a density of 2.75 times that of water be attributed, the resulting mass of the continents above sea-level is 284X10" tons. This is about one-fifth the mass of the hydro sphere or oceans.
The Lithosphere.— The lithosphere is the special province of the geologist, and is treated in detail under GEOLOGY. It is here considered, therefore, only in its broader physical aspects.
No precise value of the thickness of the shell which is called the crust can be assigned. The estimates of geologists make it 5 to _10 miles thick. It is the shell to which are confined the great rock movements and transformations with the attendant phenomena of crust crumpling, folding and faulting. For the present purposes
it may be assumed to be 10 miles thick.
In its mechanical aspects the most important fact presented by the crust is that it rests on the centrosphere, or nucleus, in substantially the same manner as a fluid crust would. In other words, the crust is essentially, in view of the forces to which it is subject, a viscous mass, which may be likened in its behavior to sealing wax. This conclusion is reached when one compares the compression to which the crust would be subjected if it were self-supporting, like a dome, with the crushing strength of rocks. That compression is about 30 times the crushing strength of the finest steel and 500 to 1,000 times that of the best building stones. Hence we must infer that at a depth of a few miles there can be no such thing as a cavity in the crust. An other conclusion of great importance, also, is that the surface shape of the earth must con form very closely to the shape it would have if it had been originally fluid, as assumed by nearly all geodesists and geologists, many of whom adduce that shape as a proof of primitive fluidity. Given time enough, and the amount of time available is ample, as seen below, the viscous earth will assume the same shape, es sentially, as a fluid earth.
Since the lithosphere, the hydrosphere and the atmosphere are the theatre of the principal activities, physical and biological, of our planet, it is of interest to know their volume, mass and chemical constitution. The volume of the shell 10 miles thick below sea-level is 1,969,400,000 cubic miles. Adding to this the volume of the crust above sea-level, namely, 22,500,000 cubic miles, there results for the volume of crust and hydrosphere down to a depth of 10 miles below sea-level 1,992,000,000 cubic miles. Of this, as stated above, 315,000,000 cubic miles are sea water, leaving for the solid matter of the crust to the depth in question 1,677,000,000 cubic miles. Attributing to this volume a density 2.75 times that of water, the mass of this solid part of the shell is 21X10" tons of 2,000 pounds. This is about 14 times the mass of the oceans. These two masses and that of the atmosphere give, in round numbers, a total of 23X10" tons, the contribution from the atmosphere being taken as 7X10' tons.