558 Dr. Gr. J. Stoney on Denudation and Deposition. 



ocean, certainly consists of superficial layers which are almost 

 exclusively oxygen salts, and of a great underlying centro- 

 sphere about which we know little. The oxygen salts have, 

 at the surface of the earth, densities which range between 

 two and three times that of water, and in forming them, along 

 with that liquid oxygen salt which we call the ocean, nearly 

 all the oxygen upon the earth is certainly used up. In proof 

 of this statement see the paper on p. 565 of this number of 

 the Phil. Mag., from which it appears that the free oxygen is 

 certainly an exceedingly small proportion of the earth's stock 

 of oxygen, probably much less than a ten- thousandth part. 

 It is farther probable that the whole would now be used up 

 in forming the oxygen salts, were it not for the ease with 

 which heat and chemical agencies evolve carbon dioxide from 

 the carbonates, and the fact that the energy that reaches 

 the earth from the sun enables vegetation to evolve free 

 oxygen over the earth's surface from the carbon dioxide so 

 produced. This enables a very small proportion of the whole 

 stock of oxygen to present itself at any one time as free 

 oxygen in contact with the outer boundary of the lithosphere. 

 From these considerations joined with others, it seems 

 probable that the oxygen salts form only a superficial crust 

 — perhaps having a thickness something like -^\-§ of the 

 earth's radius — over a relatively vast mass of deeper-seated 

 materials, which probably are not oxygen salts. In fact it is 

 probable that the great interior of the earth is for the most 

 part something intermediate between an alloy of various 

 metals and a mixture of them ; whence it is probable that 

 the materials of which it consists would, if those materials 

 were brought to standard temperature and pressure, have an 

 average density more than twice that of the layer of oxygen 

 salts. They are, however, far from being at either standard 

 temperature or pressure. What is certain about them is that 

 these materials are such and so circumstanced that the mean 

 density of the earth is little more than twice that of the film 

 of oxygen salts over its surface. The more deeply-seated 

 materials are certainly compressed by enormous pressures 

 increasing downwards, and are also as certainly so dilated by 

 the temperatures to which they are exposed, that notwith- 

 standing the extreme intensity of the compressions, the 

 counteracting expansions are great enough to bring down 

 the average of all the densities to the very low value of 5*5 

 times the density of water. 



4. Another fact w 7 hich I think we may regard as certainly 

 known, is that wherever there prevail between and within 

 the molecules of a body those active motions the energy of 



