GEOLOGICAL SYSTEMS AND PERIODS. 7 



of the earth's history during which organic life existed. Just 

 as so-called " universal history " falls into larger and smaller 

 periods, which are characterized by the conditions of de- 

 velopment of the most important nations at the respective 

 epochs, and are separated from one another by great events, 

 so we also divide the infinitely longer organic history of the 

 earth into a series of greater and less periods. Each of 

 these periods is distinguished by a characteristic flora and 

 fauna, and by the specially strong development of certain 

 vegetable or animal groups, and each is separated from the 

 preceding and succeeding period by a striking change in 

 the character of its animal and vegetable inhabitants. 



In relation to the following survey of the historical 

 course of development which the large animal and vegetable 

 tribes have passed through, it wiU be desirable to say a few 

 words first as to the systematic classification of the neptunic 

 groups of strata, and the larger and smaller periods corres- 

 ponding to them. As will be seen directly, we are able to 

 divide the whole of the sedimentary rocks lying one above 

 another into five main groups or periods, each period into 

 several subordinate groups of strata or systems, and each 

 system of strata again into still smaller groups or forma- 

 tions; finally, each formation can again be divided into 

 stages or sub-formations, and each of these again into still 

 smaller layers or beds. Each of the five great rock-groups 

 was deposited during a great division of the earth's history, 

 during a long era or eioocli ; each system during a shorter 

 period ; each formation during a stiU shorter period. In thus 

 reducing the periods of the organic history of the earth, and 

 the neptunic strata containing petrifactions deposited during 

 those periods, into a connected system, we proceed exactly 



