10 PALAEONTOLOGY OF KENTUCKY. 



ation of each species. Corresponding with those periods, the stratified rocks 

 were divided into different formations. 



The rapid progress of the geological science has expunged the division lines 

 between the periods, and has upset the theory of cataclysm. The existence of 

 different groups of fossils in different strata can not be denied ; but a more 

 careful examination of the fossils has already shown, and hereafter will show 

 more conclusively, that the existing differences are not the result of new cre- 

 ations, but are produced by the gradual modifications occurring in species 

 exposed to changes in their surroundings during the course of many centuries. 

 The theory of evolution, now generally accepted by all men of science, has 

 destroyed the theory of cataclysm, as well as the belief in an a priori creation 

 of each single species. The division of the geological time into periods, and 

 of the stratified rocks into formations, though based upon erroneous supposi- 

 tions, has proved to be of great convenience to geologists, and is, therefore, 

 maintained. The dividing lines, however, are not fixed by nature, but may 

 be shifted by arbitration. The whole geological time, from its beginning to the 

 present day, is divided into four great divisions called ages. These are as fol- 

 lows : 



1. Azoic AGE, embracing the time from the beginning of the earth as a solid 

 body to the appearance of life in the shape of either animals or plants. 



2. PAL^EOZIC AGE, beginning with the appearance of organic forms, and ter- 

 minating with the close of the coal formation. 



3. MESOZOIC AGE, beginning at the end of the coal formation, and closing at 

 the appearance of mammals. 



4. CENOZOIC AGE, embracing the balance of time up to the present day. 

 Corresponding with this division of time, we divide all the rocks into the 



four groups of azoic, palaeozoic, mesozoic and cenozoic rocks, according to the 

 age in which their respective formation took place. The first group of rocks 

 is of little or no interest to the palaeontologist, inasmuch as it does not contain 

 any traces of life ; still, what we to-day consider ^azoic, or bare of life, may, 

 to-morrow, disclose forms of undoubted organic character. Not many years 

 ago all the metamorphic rocks were considered azoic; but since Canadian geolo- 

 gists have discovered the Eozoan canadense, and have also established the fact 

 that numerous traces of coal and graphite are to be found in these rocks, the 

 existence of some kind of life during the formation of that group can not be 

 doubted any longer. As soon as the presence of life during the formation of 

 those rocks, which heretofore were classed among the azoic, is definitely settled, 

 those rocks have either to be removed to the palaeozoic group, or, as Professor 

 Dawson, of Canada, has proposed, a new group, to be called the Eozoic, has to 

 foe established for their reception, 



