GEOLOGY, 



INTRODUCTION. 



Definition of Geology. Geology is the science which 

 tieats of the past conditions of the earth and of its inhab- 

 itants. It is, therefore,, a history of the earth. It is 

 closely allied to physical geography, but differs in this : 

 Physical geography treats only of the present forms of the 

 earth's features ; geology also, and mainly of their grad- 

 ual formation, or evolution from former conditions. It is 

 also closely allied to natural history, but differs in this : 

 Natural history is concerned only with the present forms 

 and distribution of animals and plants, while geology is 

 chiefly concerned about previous forms and distribution, 

 and their changes to the present forms and distribution. 

 In a word, geography and natural history are concerned 

 about how things are ; geology, about how they became so. 



Cultivates Habit of Observation. We have said 

 geology treats of the history of the past conditions of the 

 earth and its inhabitants. The evidences of the past con- 

 ditions are found in its present structure. But, to under- 

 stand this structure, we must observe the manner in which 

 similar structure is formed now under our eyes. Thus, 

 observation of causes noiv in operation constitutes the 

 only solid foundation of geology. Fortunately, the proc- 

 esses by which structure is now being formed may be ob- 

 served everywhere ; and the structures which have been 



