4O MANUAL OF ZOOLOGY. 



which we are able to examine directly. The following are the 

 chief facts which it is necessary for the student to bear in mind : 



1. The rocks which compose the crust of the earth have 

 been formed at successive periods, and may be roughly di- 

 vided into aqueous or sedimentary rocks, and igneous rocks. 



2. The igneous rocks are produced by the agency of heat, 

 are mostly unstratified (i.e., are not deposited in distinct layers 

 or strata), and, with few exceptions, are destitute of any traces 

 of past life. 



3. The sedimentary or aqueous rocks owe their origin to 

 the action of water, are stratified (i.e., consist of separate layers 

 or strata), and mostly exhibit " fossils " that is to say, the 

 remains or traces of animals or plants which were in existence 

 at the time when the rocks were deposited. 



4. The series of aqueous rocks is capable of being divided 

 into a number of definite groups of strata, which are technically 

 called " formations." 



5. Each of these definite rock-groups, or "formations," is 

 characterised by the occurrence of an assemblage of fossil re- 

 mains more or less peculiar and confined to itself. 



6. The majority of these fossil forms are " extinct ;" that is 

 to say, they do not admit of being referred to any species at 

 present existing. 



7. No fossil, however, is known, which cannot be referred 

 to one or other of the primary subdivisions of the Animal 

 Kingdom, which are represented at the present day. 



8. When a species has once died out it never reappears. 



9. The older the formation, the greater is the divergence 

 between its fossils and the animals and plants now existing on 

 the globe. 



10. All the known formations are divided into three great 

 groups, termed respectively Palaeozoic or Primary, Mesozoic 

 or Secondary, and Kainozoic or Tertiary. 



The Palaeozoic or Ancient-life period is the oldest, and is 

 characterised by the marked divergence of the life of the period 

 from all existing forms. 



In the Mesozoic or Middle-life period, the general fades of 

 the fossils approaches more nearly to that of our existing fauna 

 and flora; but with very few exceptions the characteristic 

 fossils are all specifically distinct from all existing forms. 



In the Kainozoic or New-life period, the approximation of 

 the fossil remains to existing living beings is still closer, and 

 some of the forms are now specifically identical with recent 

 species ; the number of these increasing rapidly as we ascend 

 from the lowest Kainozoic deposit to the Recent period. 



