ZOOLOGICAL TESTS. 



previously to fix the sense .of certain conventional terms necessary 

 to be used in such a narrative, by which the effects of the 

 succession of terrestrial convulsions, and the intervals of geological 

 time will be expressed. 



236. As has been already stated, the earth we inhabit has 

 undergone a long series of convulsions, affecting its entire sur- 

 face, each of which has been succeeded by a long interval of 

 tranquillity, during which, the waters, in a state of equilibrium 

 and quiescence, deposited the solid matter suspended in them in a 

 series of layers, and in each of these intervals the author of 

 nature called into existence an animal and vegetable kingdom, 

 resembling more or less the present. 



237. The intervals of time which elapsed thus between convul- 

 sion and convulsion, we shall call geological periods, and the 

 mineral strata deposited by the waters during such periods, we 

 shall call geological stages. 



Unlike historic, geological time is therefore not measured by 

 years and centuries. Its units are much more vast. Each of 

 them is a period, that is an interval of time, whose exact length 

 is unascertained, but which must be considered as having an 

 analogy, more or less close, to the interval which will have elapsed 

 between the creation of the present animal kingdom and the epoch, 

 whenever it may arrive, at which, it, like all those which pre- 

 existed, shall be swept away. 



238. An analysis of the strata composing the crust of the earth 

 has presented certain features, upon which geologists have founded 

 a classification of the stages just mentioned. According to this 

 classification, the stages, from the igneous rocks to the latest 

 deposits, which immediately preceded the appearance of the present 

 state of things, have been resolved into six groups, denominated 

 as follows : 



1. Azoic formation. 



2. Palaeozoic formation. 



3. Triassic formation. 



4. Jurassic formation. 



5. Cretaceous formation. 



6. Tertiary formation. 



239. The Azoic formation consists of groups of strata, the lowest 

 of which reposes upon the igneous rocks. The Palaeozoic formation 

 rests upon the uppermost strata of the Azoic group, and in it, as its 

 name implies, are found the first traces of organic life. The general 

 stratigraphic characters of the other groups may be seen by 

 reference to the tabular sections of the Earth's Crust, given in 

 47. 



The Palaeozoic formation consists of five distinct stages, each 



29 



