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CHAPTER XVII. 



THE GENERAL APPLICATIONS OF GEOLOGY TO 

 AGRICULTURE, 



Changes that the earth's surface has undergone. Unstratified or 

 primary rocks. Stratified or secondary and tertiary rocks. 

 Regular succession of the strata. Each stratum known by 

 characteristic fossils. Composition of granite, and of traps or 

 basalts. Differences in cortiposition among the stratified rocks. 

 " Illustration by diagram. Of disturbing causes which have al- 

 tered soils. Drift; explanation of its nature, and theories of 

 its formation. Alluvial deposits. Practical advantages of 

 geological knowledge. 



SECTION I. OF THE STRATIFED AND UNSTRATIFIED ROCKS. 



Geological science explores the structure of this 

 earth's surface, to as great a depth as our means of 

 observation extend. In the course of geological in- 

 vestigations, various important and interesting laws 

 have been established. 



It is found that the earth has been, before man 

 inhabited it, a scene of constant change and convul- 

 sion. Forces from w^ithin and without, have elevated, 

 upheaved, and even overturned, some portions of its 

 surface; while others have been overwhelmed, or de- 

 pressed, in a corresponding degree. Dry land has 

 thus appeared where seas had flowed, and seas have 

 swept over what had long been elevated above their 

 surface. But it may be asked, how do we know all 

 of these facts 1 The answer to this is plain : simply 

 by investigation of existing rocks, in the phenomena 

 connected with their position and structure. 



