28 THE EARTH FORMED, ETC. 



tary rock series to its close, the mineral masses are found to 

 enclose remains of the organic beings (plants and animals) 

 which flourished upon earth during the time when the various 

 strata were forming ; and these organisms, or such parts of 

 them as were of sufficient solidity, have, in many instances, 

 been preserved with the utmost fidelity, although for the most 

 part converted into the substance of the enclosing mineral. 

 Now, as we pass along through the series of strata, we find a 

 cessation of certain specific forms of plants and animals, while 

 others come into view ; at some points, the change is almost 

 complete at others, it is very considerable. Such demarca- 

 tions are taken into account by geologists in the grouping of 

 the rock series. They speak of a Palceozoic Period, compre- 

 hending a large early section, terminating at a point where 

 the specific forms are for the first time almost wholly changed ; 

 a Secondary Period, and a Tertiary Period, these also giving 

 groups of species all but distinct. Under each period are 

 reckoned certain systems, more or less organically distinct, 

 and these we shall now proceed to treat separately. 



