INTRODUCTION 7 



and basalt, the structure of which is due to the action of the 

 internal heat of the globe, and which originate below the surface 

 and are not arranged in layers or strata ; (2) Aqueous or sedimentary 

 rod's, which arise by the disintegration, at the surface of the earth, 

 of pre-existing rocks, the fragments or debris being carried off by 

 streams and rivers and deposited at the bottom of lakes or seas. 

 Being formed in this way by the deposition of successive layers or 

 strata, the sedimentary rocks have a stratified structure, the lowest 

 being in every case older than the more superficial layers. The 

 researches of geologists have shown that there is a general order of 

 succession of stratified rocks : that they may be divided into three 

 great groups, each representing an era of time of immense but 

 unknown duration, and that each group may be subdivided into 

 more or fewer systems of rocks, each representing a lesser period of 

 time. The following table shows the thirteen rock-systems usually 

 recognised, arranged under the three great groups in chronological 

 order, the oldest being at the bottom of the list. 



i!3. Quaternary and Recent. 



U. Sbcene 6 : 



10. Eocene. 



j 9= Cretaceous. 



II. Mesozoic or Secondary . s 8. Jurassic. 



I 7. Triassic. 



6. Permian. 



I. Palaeozoic or Primary . . 



5. Carboniferous. 

 4. Devonian. 

 3. Silurian. 



2. Cambrian. 

 1. Laurent ian. 



Imbedded in these rocks are found the remains of various extinct 

 animals in the form of what are called fossils, In the more recent 

 rocks the resemblance of these to the hard parts of existing 

 animals is perfectly clear : we find shells hardly differing from 

 those we pick up on the beach, bones easily recognisable as those 

 of Mammals, Birds, or Fishes, and so on. But in the older rocks the 

 fossils are in many cases so different in character from the animals 

 existing at the present day as to be referable to no existing order. 

 We find Birds with teeth, great aquatic Reptiles as large as Whales, 

 Fishes, Molluscs, Crustacea, &c., all of an entirely different type from 

 any now existing. We thus find that the former were in many 

 cases utterly unlike the present animal inhabitants of the globe, 

 and we arrive at the notion of a succession of life in time^ and are 

 even able, in exceptionally favourable circumstances, to trace back 

 existing forms to their extinct ancestors. 



By combining the results of comparative morphology, embryology, 



