XIII THE GEOLOGICAL EVIDENCES OF EVOLUTION 379 



Causes of the Imperfection of the Geological Record. 



These facts are quite in accordance "with the conclusions of 

 geologists as to the necessary imperfection of the geological 

 record, since it requires the concurrence of a number of 

 favourable conditions to preserve any adecjuate representation 

 of the life of a given epoch. In the first place, the animals to 

 be preserved must not die a natural death by disease, or old 

 age, or by being the prey of other animals, but must be 

 destroyed by some accident which shall lead to their being 

 embedded in the soil. They must be either carried away by 

 floods, sink into bogs or quicksands, or be enveloped in the 

 mud or ashes of a volcanic eruption ; and when thus embedded 

 they must remain undisturbed amid all the future changes of 

 the earth's surface. 



But the chances against this are enormous, because de- 

 nudation is always going on, and the rocks we now find at 

 the earth's siu'face are only a small fragment of those which 

 were originally laid down. The alternations of marine and 

 freshwater deposits, and the frequent unconformability of 

 strata with those which overlie them, tell us plainly of 

 repeated elevations and depressions of the surface, and of 

 denudation on an enormous scale. Almost every mountain 

 range, with its peaks, ridges, and valleys, is but the remnant 

 of some vast plateau eaten away by sub-aerial agencies ; every 

 range of sea-clifFs tell us of long slopes of land destroyed by 

 the waves ; while almost all the older rocks which now form 

 the surface of the earth have been once covered with newer 

 deposits which have long since disappeared. Nowhere are 

 the evidences of this denudation more apparent than in North 

 and South America, where granitic or metamorj^hic rocks cover 

 an area hardly less than that of all Europe. The same rocks 

 are largely developed in Central Africa and Eastern Asia ; 

 while, besides those portions that appear exj^osed on the 

 siu-face, areas of unknown extent are buried under strata 

 which rest on them uncomformably, and could not, there- 

 fore, constitute the original capping under which the whole of 

 these rocks must once have been deeply buried ; because 

 gi'anite can only be formed, and metamorphism can only go 

 on, deep dowTi in the crust of the earth. What an over- 



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