30 GEOLOGICAL HISTORY. [CH. 



folding, as such phenomena are by no means exceptional in many 

 parts of the world. 



The order of superposition of strata has afforded the key to 

 our knowledge of the succession of life in geologic time, and the 

 refinements of the strati graphical correlation of sedimentary 

 rocks are based on the comparison of their fossil contents. By 

 a careful examination of the relics of fossil organisms obtained 

 from rocks of all ages and countries', it has been found possible 

 to restore in broken outline the past history of the Earth. 

 By means, then, of stratigraphical and palaeontological evidence, 

 a classification of the various rocks has been established, the 

 lines of division being drawn in such places as represent gaps 

 in the fossil records, or striking and widespread unconformities 

 between different series of deposits. 



It is only in a few regions that we find rocks which can 

 reasonably be regarded as the foundation stones of the Earth. 

 As the globe gradually cooled, and its molten mass became 

 skinned over with a solid crust, crystalline rocks must have 

 been produced before the dawn of life, and before water could 

 remain in a liquid form on the rocky surface. As soon as the 

 temperature became sufficiently low, running water and rain 

 began the work of denudation and rock disintegration which 

 has been ceaselessly carried on ever since. In this continual 

 breaking down and building up of the Earth's surface, it would 

 be no wonder if but few remnants were left of the first formed 

 sediments of the earliest age. 



The action of heat, pressure and chemical change accom- 

 panying rock-foldings and crust-wrinklings, often so far alters 

 sedimentary deposits, that their original form is entirely lost, 

 and sandstone, shales and limestones become metamorphosed 

 into crystalline quartzites, slates and marbles. 



The operation of metamorphism is therefore another serious 

 difficulty in the way of recognising the oldest rocks. The 

 earliest animals and plants which have been discovered are 

 not such as we should expect to find as examples of the first 

 products of organic life. Below the oldest known fossiliferous 

 rocks, there must have been thousands of feet of sedimentary 

 material, which has either been altered beyond recognition, or 



