THE CHILDHOOD OF THE EARTH 



to cool, and going down in temperature to 115, gave 

 opportunity to animal life. At the end of this era the 

 general temperature of the earth and its oceans was 

 as low as 90 F. The first animal life had begun to 

 appear; its activity greatly increased under what were 

 favourable conditions for it. This increase of animal life 

 had its effect on the earth's crust. We have already 

 spoken of the formation of limestones from the bodies 

 of sea animals. This was going on in those ages millions 

 of years ago before any of the higher forms of life had 

 appeared on the earth, and though it was not going 

 on so rapidly, still it must be remembered that at some 

 point of the world's history the oceans were of greater 

 extent than now, and consequently the deposits of lime 

 and the accumulations of sediment were more widespread. 

 The sedimentary rocks grew faster and faster, especially 

 on the floors of the oceans. 



It will be understood by those who have read the 

 foregoing two chapters closely that the " igneous " or 

 fire-born rocks must lie underneath the sedimentary ones. 

 But that is only true in general terms, for a double 

 reason. In the first place, owing to the inestimable 

 forces which for millions of years were still continually 

 effective below the earth's crust, the igneous rocks over 

 and over again were able to burst their way through 

 the slow-forming sediment of other rocks laid down 

 above them. In the second place, the igneous rocks, 

 owing to their composition and superior hardness, were 

 much less worn by wind and weather than the less com- 

 pact "sedimentary rocks," and these remained, showing 



104 



