88 DOCTRINE OF EVOLUTION 



geological agencies. And at the same time that old 

 strata are undergoing destruction new ones are in 

 process of construction at other places, where animal 

 remains can be embedded and preserved as fossils. The 

 forces at work seem weak, but they continue their opera- 

 tions through ages that are beyond our comprehension 

 and they accomplish results of world-building magnitude. 

 Thus the whole process of geological construction is 

 such that older exposed strata continually undergo 

 disintegration, but this involves the destruction of any 

 fossils that they might contain. The very forces that 

 preserve the relics of extinct animals at one time undo 

 their work at a later period. There are many other 

 influences besides that destroy the regularity of rock 

 layers or change their mineralogical characters by meta- 

 morphosis. It is easier to see how volcanic outbursts 

 alter their neighboring territory. The intense sub- 

 terranean heat and imprisoned steam melt the deeper 

 substances of the earth's crust, so that these materials 

 boil out, as it were, where the pressure is greatest, and 

 where lines of fracture and lesser resistance can be found. 

 Because so much detritus is annually added to the 

 ocean floors — enough to raise the levels of the oceans 

 by inches in a century — it is natural that greater 

 pressures should be exerted in these areas than in the 

 slowly thinning continental regions. These are some 

 of the reasons why volcanoes arise almost invariably 

 along the shores or from the floors of great ocean beds. 

 The chain that extends from Alaska to Chili within the 

 eastern shore of the Pacific Ocean, and the many hun- 

 dreds of volcanoes of the Pacific Islands bring toi the 

 surface vast quantities of eruptive rocks which break 



