PART II. HISTORICAL GEOLOGY 

 CHAPTER XIII 



HISTORICAL GEOLOGY 



Historical geology deals with the evolution of the life of the past, 

 and with the development of the continents and oceans. It traces 

 out, as accurately as our present knowledge will permit, the changes 

 through which the earth has passed ; it endeavors to gather from 

 the available record the history of the life of geological times and 

 the evolutional changes which the many classes of animals and plants 

 have undergone and, as far as possible, to determine the cause or 

 causes of these changes. This section of geology is concerned not 

 only with the recording of facts, but is also, to an important degree, 

 philosophical. 



Human history is but a short chapter of geological history, the 

 former being measured in thousands of years while the latter extends 

 over millions of years. The immensity of geological time is beyond 

 our comprehension, but some conception of it can be gained when it 

 is remembered that the time necessary to excavate the Grand Canyon 

 of the Colorado was, geologically, comparatively short; that a 

 maximum thickness of sediments of not less than 40 miles has been 

 laid down in the seas; that great mountain ranges have not only 

 been raised but have been worn down to sea level during portions 

 of the smaller divisions of geological history. Perhaps the most 

 striking evidence of the length of geological time is to be seen in the 

 evolution of life. 



Fossils 



A fossil is any remains or trace of an animal or plant preserved in 

 the rocks of the earth. It may consist of the original substance of 

 the animal, or it may be merely an impression, such as a footprint 

 or a worm trail. Even the flint implements made by primitive man 

 may be considered as fossils. 



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