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CHAPTER I. 



MODES OF FORMATION AND GENERAL CLASSIFICATION OF 

 ROCKS, AQUEOUS AND IGNEOUS. 



IN old days, those who thought upon the subject at all 

 were content to accept the world as it is, believing that 

 from the beginning to the present day it had always 

 been much as we now find it, and that, till the end of 

 all things shall arrive, it will, with but slight modifica- 

 tions, remain the same. 



But, by and by, when Geology began to arrive at 

 the dignity of a science, it was found that the world had 

 passed through many changes ; that the time was when 

 the present continents and islands were not, for the 

 strata and volcanic products of which both are formed 

 were themselves sediments derived from the waste of 

 yet older lands now partly lost to our knowledge, or 

 of newer accretions of volcanic matter erupted from 

 below. Thus it happens that what is now land has 

 often been sea, and where the sea now rolls has often 

 been land ; and that there was a time before existing 

 continents and islands had their places on the earth, 

 before our present rivers began to flow, and before all 

 the lakes of the world, as we now know them, had begun 

 to be. 



Geology may therefore be defined as the science 

 which investigates the history of the earth, or the 

 successive changes which have taken place in the in- 



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