x EARTH CHANGES & EVOLUTION 179 



skeletons of minute organisms, together with small quantities 

 of decomposed pumice and of meteoric or volcanic dust. 

 Along with these in certain areas the remains of larger 

 marine animals are found, especially the otoliths (or ear bones) 

 of whales and the teeth of sharks. And the extreme slow- 

 ness of the deposit of these oozes is shown by the fact that 

 it is often impossible to bring up a dredging from the bottom 

 that does not contain some of these bones or teeth. It 

 seems as if much of the ocean bed were strewn with them ! 

 Now, these oozes, so easily recognised by their component 

 materials and their organic remains, form no part of the 

 upheaved crust of the earth on any of our continents. This 

 is, of itself, a conclusive proof that oceans and continents 

 have never changed places in the whole course of known 

 geological time ; for if they had done so (as is still main- 

 tained by many rather illogical writers) the epoch of sub- 

 mergence would be indicated by some fragments, at least, of 

 the consolidated ocean ooze which must once have covered 

 the whole continental area. 1 



Thickness of the Earth's Crust 



We now have to consider a quite different set of pheno- 

 mena which have a very important bearing on the causes 

 which have produced the elevations and depressions which 

 have occurred over much of the land surface of the globe. 

 It is a universal fact that as we descend into the crust of 

 the earth (in deep wells or mines) the temperature rises at a 

 tolerably uniform rate, which is found to be on the average 

 one degree Fahr. for every 47^ feet. This rate, if continued 

 downwards, would reach the temperature of melted rock at 

 a depth of about 20 miles. Hot springs in non- volcanic 

 countries furnish an additional proof of the high temperature 

 of the interior. Below the depth above indicated there 

 would probably be some miles of rock in a plastic state, 

 while irregularities would result from the nature of the rock, 

 some being more easily melted than others. 



1 For a full discussion of this question see my Darwinism, chap. xii. ; Island 

 Life, chaps, vi. and x. ; and Studies Scientific and Social, vol. i. chap. ii. In 

 this last work the whole argument is summarised and the numerous converging 

 lines of evidence pointed out. 



