The Lithosphere 



original, but frequently retaining much of its structure, 

 are known as fossils. William Smith's great discovery 

 was that the assemblage of fossils in a given group 

 of rocks was always the same, even when it occurred 

 in widely separated areas, while the remains in the group 

 above were different from these, but again constant among 

 themselves. 



Thus the rocks of the different geological periods 

 can be identified by their fossil contents, and the rocks 

 of the different continents linked up and correlated one 

 with another. 



Charles Darwin and Alfred Russel Wallace, by their 

 independent discovery of the principle of evolution, 

 furnished the explanation of this arrangement of the 

 fossils; the fossil fauna of each bed consisting of the 

 descendants of those below, and being the ancestors of 

 those in the layer above, the time taken for the formation 

 of a group of rocks being sufficiently great to allow of 

 considerable variation in the forms of life. 



As an introduction to the chapters which are to 

 follow, a brief account of the geological formations and 

 of the fossils which they contain is here given for the 

 use of those readers who have no previous knowledge of 

 geology. 



In the first place, the stratified or sedimentary rocks 

 have been divided in three groups Palaeozoic, Mesozoic, 

 and Cainozoic. 



The Palaeozoic rocks are those which contain the 

 remains of ancient forms of life, many of the animals 

 having been of very primitive types and having become 

 extinct at the close of the Palaeozoic Era. 



The Mesozoic Era was characterized by higher forms 



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