igoy.j AND MOUNTAIN FORMATION. 413 



bottom, as shown by the accompanying- seismic sea wave, so fre- 

 quently occurs during great earthquakes. 



The force of the argument that the ridge was uplifted by matter 

 actually expelled from under the trench is easily seen from a 

 familiar illustration often met wath in a smooth field or on the plains. 

 If we go along over the level ground, and all of a sudden come upon 

 a mound with a sink hole near it of about the same volume, we 

 immediately conclude that the mound builders or pond diggers have 

 been at work. It is obvious that the mound is made of soil which 

 came out of the hole. In the same way, if we came upon a ditch 

 with a ridge to one side, of such volume that it would about fill up 

 the former, we know with even greater certainty that ditch diggers 

 have taken the soil out of the trench and piled it upon the bank. 



Now the trenches in the sea bottom, with parallel ridges of nearly 

 equal volume, have all the force of this familiar argument; but as 

 they have not been dug from above, it follows that the matter is 

 transferred beneath the crust, by subterranean bodily movement of 

 the kind we have described ; and hence the crust has been crumpled 

 correspondingly, the ridge running parallel to the trench from which 

 the matter was expelled. 



§24. On Darzvin's Theory of Coral Islands. — This investigation 

 throws decided light upon Charles Darwin's famous theory of the 

 sinking of the ocean bottom, as inferred from the study of the coral 

 reefs. It shows that while sinking often takes place, elevation also 

 is very common. The chief difference is that elevation kills the 

 coral, and the islands gradually become covered with vegetation, 

 while only those in which sinking predominates become surrounded 

 by coral reefs. It is well known that there are many islands in the 

 sea with coral rock above the water ; a good illustration is afforded 

 by the eastern end of Guam, where a huge block has been uplifted, 

 as shown by the vertical walls. Islands which have been recently 

 uplifted often are volcanic, so that coral appears especially about old 

 islands, many of which are subsiding, owing to a change in the 

 direction of movement beneath the crust. Forces which formerly 

 produced elevation are now changed in direction and working so as 

 to produce a slow subsidence. Charles Darwin's theory of coral 

 reefs is therefore confirmed, but it does not explain all the phe- 



