THE STORY OF THE EARTH 39 



toward the land surface to compensate the increased 

 burden. This would be met by a stubborn resistance, 

 and in consequence a vast mass of rock in the shore 

 region would be slowly lifted up above the general level, 

 and form a mountain ridge. A glance at the map of 

 America, showing the great chain of mountains along 

 the Pacific coast, will at once provide an illustration of 

 the theory. The shrinkage of the earth and necessary 

 crumpling of the crust is not, of course, called in ques- 

 tion ; but the additional theory gives a more satisfactory 

 idea of the formation of mountains. 



This process, however it be conceived, began in the 

 Devonian period, and gave us our oldest hills. The 

 greater mountain chains belong, as we shall see, to a 

 very much later date. But the Devonian period was 

 also characterised by a gradual depression of the crust 

 at the continental surfaces, so that great arms of the sea 

 penetrated inland, and ultimately formed a series of vast 

 lakes. For English people the Devonian is mainly the 

 age of the "Old Red Sandstone" rocks, which form so 

 conspicuous a feature of the Devon coast. The dark red 

 colour of these rocks is due to the deposit of iron round 

 the grains of sand that compose them, and this points 

 to a formation in inland lakes rather than the open sea. 

 This change coincides with the first appearance of 

 animals that live on land, and the connection of 

 the two will be considered in a later chapter. The 

 earth has been, not merely the passive theatre of the 

 upward progress of life, but the great stimulus to its 

 progress ; and it is well to establish the evolution of the 

 earth itself, on geological grounds, before we see the 

 bearing of its changes on the procession of living things. 

 The Carboniferous period, which succeeded the 

 Devonian, is probably the one best known to the 

 general reader, and requires little description. After the 



