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 adequate 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 bo 
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 surface 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 metamorphic 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 exposed on the 
surface, 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 
granite can only be formed, and metamorphism can only go 
on, deep down in the crust of the earth. What an over- 
