12 EVOLUTION OF LIFE 1.9 



and carries away the surface of the land, at a rate of the order of 1 ft 

 per 4,000 years, the processes known as weathering and denudation. 

 The material carried away is deposited in the river-beds and in the 

 lakes and shallow seas around the river mouths (sedimentation) (Fig. 

 1). Here it builds the sedimentary rocks, which may be many thou- 

 sands of feet in thickness, the whole continental platform continuing 

 to sink for long periods, perhaps with intervals during which it be- 

 comes raised above the water. Fossil remains are usually the result of 



km 

 8 



6- 

 4- 



8,840m. 



High mountain 

 ranges 



Average height ^ 

 r of land tr X$ 



\0km 



Fig. i. Curve showing the areas of the earth's solid surface in relation to the 

 sea level. (From Holmes.) 



the preservation of the harder parts of animals in sedimentary deposits, 

 and the most complete series of fossils are likely to be those of animals 

 living in the seas. 



The surface crust of the earth is not a layer of uniform thickness and 

 density but consists of irregular masses of lighter material, rich in 

 silicon and aluminium (sial), forming the continents, and heavier 

 material, rich in magnesium (sima), under the ocean beds. The reason 

 for this non-uniform distribution is obscure, but it has the effect of 

 making the continents stand higher, floating on the plastic denser 

 medium beneath the crust. When material is removed from the con- 

 tinents by denudation they rise; conversely the addition of millions 

 of tons of ice will depress them. The continents are thus said to rest 

 in isostatic equilibrium, and following the small changes in level the 

 sea leaves more or less of the continental shelf uncovered. Such 

 upward and downward movements profoundly influence the climate. 

 Oceanic climatic influences tend to produce a damp, equable climate, 

 with large areas of marsh and forest. When the land stands higher 



