PART III. 

 HISTORICAL GEOLOGY. 



CHAPTER I. 



GEKERAL PRINCIPLES. 



GEOLOGY is essentially a history. But there are two 

 points of view from which history may be studied, viz. : 1. 

 As a chronicle of thrilling events. 2. As the science of 

 the laws of succession, and of the causes of these events. 

 The interest in the one case is dramatic, in the other, 

 scientific. The one addresses itself mainly to the imagi- 

 nation, the other to the reason. It is almost unnecessary 

 to say that geology is a history in which the second ele- 

 ment predominates. It is a history of the evolution of 

 the earth and of its inhabitants. Now, there are certain 

 general laws of evolution in all departments of nature 

 certain general principles underlying all history. The 

 most important of these we wish to fix in the mind of 

 the pupil by comparing geology with human history : 



1. Human history is divided and subdivided into eras, 

 ages, periods, epochs, etc., determined by great events. 

 These divisions of time are recorded in separate volumes, 

 chapters, sections, etc., according to their importance. 

 So, also, the history of the earth is divided into eras, ages, 

 periods, etc., determined by great changes in physical 

 geography, climate, and forms of organisms ; and these 

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