INTEODUOTOET. 



DEFINITION OF GEOLOGY, AND OF ITS DEPAPTMENTS. 



Geology is the physical history of the earth and its inhabitants, 

 as recorded in its structure. It includes an account of the changes 

 through which they have passed, the laws of these changes, and their 

 causes. In a word, it is the history of the evolution of the earth and 

 its inhabitants. 



The fundamental idea of geology, as well as its principal sub- 

 divisions and its objects, may be most clearly brought out by compar- 

 ing it with organic science. We may study an organism from three 

 distinct points of view : 1. We may study its general form, the parts 

 of which it is composed, and its minute internal structure. This is 

 anatomy. It is best studied in the dead body. 2. We may study the 

 living body in action, the function of each organ, the circulation of the 

 fluids, and the manner in which all contribute to the complex phenom- 

 ena of life. This is physiology. 3. We may study the living and 

 growing body, by watching the process of development from the egg 

 to the adult state, and striving to determine its laws. This is embry- 

 ology. 



So, looking upon the earth as an organic unit, we may study its 

 form, the rocks and minerals of which it is composed, and the manner 

 in which these are arranged ; in other words, its external form and in- 

 ternal structure. This is the anatomy of the earth, and is called struct- 

 ural geology. Or, we may study the earth under the action of physical 

 and chemical forces, the action and reaction of land and water, of earth 

 and air, and the effects of these upon the form and structure. This is 

 the physiology of the earth, and is called dynamical geology. Finally, 

 we may study the earth in the progress of its development, from the 

 earliest chaotic condition to its present condition as the abode of man, 

 and attempt to determine the laws of this development. This is the 

 embryology of the earth, or historical geology. 



Principal Departments. — The science of geology, therefore, nat- 

 urally divides itself into three parts, viz. : 1. Structural geology, or 

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