Denudation and Deposition. 373 



compressibility of the lithosphere and centrosphere taken 

 together is not far from that of the more incompressible kinds 

 of glass. Glass of this description yields to compression 

 about 2^ times more than solid cast iron, but less than 

 mercury (which seems to be the only liquid metal that has 

 been experimented on) in the ratio of 2 to 3. It is about 

 20 times more incompressible than water. 



We shall then, as a provisional hypothesis, assume that the 

 earth has the same compressibility as the more resistant kinds 

 of glass, which lose about 2J billionths of their volume for 

 each pressure of a dyne per square centimetre over their 

 surface. Combining this with the volume of the earth- 

 pyramid given above, we find that our hypothesis leads to the 

 conclusion that if the sides of the pyramid were kept from 

 yielding, and if the weight of a cubic centimetre of water 

 were placed on its outer end, this would reduce its hulk by 

 half a cubic centimetre. A cubic centimetre of stone, of 

 specific gravity 3, would accordingly depress its outer end by 

 1-J- centimetres. It follows from this that if meteors rained 

 upon the earth (supposed to be without an ocean) producing 

 a deposit over its whole surface a centimetre thick, and of 

 material as dense as stone., the result would be that the earth 

 after this accession would be smaller ; its surface would sink 

 down about half a centimetre. Correspondingly, if by any 

 agency a centimetre of the earth's crust could be removed over 

 the whole earth, the earth's surface would stand J a centimetre 

 higher than before. These are the effects which deposition and 

 denudation would respectively produce if they could operate 

 over the whole earth. And, if they operate over any extensive 

 area of the earth's surface ; they will produce effects of the same 

 kind, complicated a little by the displacement of the earth's 

 centre of attraction, or rather locus of centres of attraction. 



This may be well seen in the oldest parts of the oldest 

 continents — parts of Asia and Africa — to whose present 

 elevation denudation *, operating over an extensive area and 

 for long ages, has probably chiefly contributed. And, corre- 

 spondingly, there is a deepening of those parts of the ocean 

 where the deposition of sufficiently heavyt material has been 

 going on over a great area for an immense time. 



* Underground waters produce the same dynamical eifect as surface 

 denudation, by reason of the materials they remove in solution. 



f Yv^here the sub-aqueous deposit is spread over only a small part of 

 the surface of the globe (which is the only case we need consider), the 

 compression is due, not to the whole weight of the deposit, but only to 

 its excess over the weight of an equal bulk of water. Hence to produce 

 an equivalent effect the material must be denser than it would need to 

 be if the deposition had been on land. 



