38 EVOLUTION 



After the Silurian period are two of great interest in 

 regard to the story of evolution. The Devonian period, 

 which immediately followed it, witnessed the formation 

 of the first great mountain chains and the first system of 

 lakes. At the beginning of the period the land rose 

 slowly into mountain ridges in parts of North America 

 and in Scotland and Scandinavia. The cause of the 

 formation of mountain chains has been usually described 

 as a crumpling of the earth's crust on account of the 

 shrinkage of the inner body of the planet. At its first 

 formation the skin would tend to be too small for the 

 earth's body. It would crack into great slabs and sink in 

 the molten mass. After the cooling of the body had 

 proceeded for some further millions of years, the rigid 

 skin, unable to adapt itself to the shrunken body, would 

 shrink into folds and creases. One may recall the 

 wrinkling of the skin on the attenuated frame of an old 

 man. Imagine the skin wrinkling outwards, instead of 

 inwards, and one has a good idea of the general concep- 

 tion of the rise of mountain ridges. This facile theory 

 has, however, been called in question of late years. 

 Professor Sollas, for instance, believes that the rise of 

 the crust into mountains is rather connected with the 

 laying of tons of sediment on the floor of the ocean. The 

 river system of England is carrying, year by year, thou- 

 sands of tons of matter off the face of the country, and 

 depositing it on the floor of the sea. The deposit off the 

 coast of America is far greater. After two or three 

 million years of such deposition there will be a terrific 

 burden pressing on the crust under the ocean, and a 

 corresponding relief of pressure on the land. The result 

 will be a displacement of rock from below the deposited 

 strata and toward the land. The pressure of the crust 

 of the earth is so great that at five miles from the 

 surface the rocks are probably plastic, and would flow 



