INTRODUCTION 



Geology is the science that treats of the earth and its inhabitants 

 as revealed in the rocks, and therefore deals with its constitution and 

 structure, with the operation of the forces which led to its present 

 condition, and with the occurrence and evolution of its life. In the 

 search for this knowledge it calls to its aid astronomy, chemistry, 

 physics, and biology. Geology is, in fact, a composite science, making 

 use of the physical sciences in unrolling the complicated history and 

 structure of this planet. 



Because of the breadth of its scope, geology has been divided into 

 a number of branches which are, however, in such a large measure 

 interdependent, that a general knowledge of all is often essential to 

 a thorough understanding of any one. 



Astronomic or Cosmic Geology. — Since the earth is one of the 

 planets of the solar system, all theories of its origin must at the same 

 time consider the origin of the other planets, and vice versa. Con- 

 sequently, astronomy and geology are dependent upon one another 

 in all attempts at determining the genesis of the earth. 



Structural Geology is a study of the materials of which the earth 

 is built and their arrangement, and is especially concerned with the 

 interpretation of the structures produced in the rocks by earth 

 movements. The branch of geology which investigates minerals is 

 mineralogy, and that which deals with rocks is petrology (Greek, 

 petros, rock, and logos, discourse). Both mineralogy and petrology 

 are closely allied to chemistry and optical physics. 



Dynamical Geology is a study of the agencies that have produced 

 geological changes, together with their laws and modes of operation. 

 Among the most important forces considered are water in motion, 

 wind, glaciers, igneous activity, and earth movements resulting from 

 strains. This branch of the subject is closely related to physio- 

 graphic geology, since the latter deals with the evolution of the 

 topography of the earth's surface and with the forces which have 

 produced it. A study of physiographic geology is necessary to an 

 interpretation of land surfaces. 



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