HISTORICAL GEOLOGY 



WITH SPECIAL REFERENCE TO NORTH 

 AMERICA 



CHAPTER I 



GENERAL PRINCIPLES 



What Historical Geology Teaches 



Historical geology deals with the evolution of earth struc- 

 tures and organisms. Its object is to arrange the events of earth 

 history in the regular order of their occurrence. The records of 

 these events are preserved in the rocks of the crust of the earth, 

 the layers (strata) of which have been likened to the leaves of a 

 great book. Many times the pages of this vast "nature-book" 

 contain remarkable records and illustrations, while at other 

 times they are comparatively barren. In order that the reader 

 may, at the outset, form some general idea of the scope and char- 

 acter of the subject, the following summary of the more important 

 conclusions derived from the study of earth history is here presented. 



Inorganic Inferences. — 1. The age of the earth must be meas- 

 ured by at least tens of millions of years. One great mountain range 

 after another has been built up and then worn away by the ordi- 

 nary processes of erosion. Many thousands of feet (in thickness) 

 of strata have been accumulated by the deposition of sediments 

 slowly derived by the removal of thousands of feet of materials 

 from the lands. Such facts force us to the inference of a vast 

 antiquity for the earth. 



2. The physical geography of the earth has been notably different 

 in earlier geological time from that of the present. For example, 

 many millions of years ago (during the Ordovician period) an inte- 

 rior sea spread over much of what are now the Mississippi Valley 

 and the Appalachian Mountain regions, as well as regions still 

 farther westward. 



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