PART I 



DYNAMICAL GEOLOGY 



We have already seen that the chief task of geology is to con- 

 struct a history of the earth, to determine how and in what order 

 the rocks were formed, through what changes they have passed, 

 and how they reached their present position. The logical order 

 of treatment might seem to require that we should first learn what 

 the rocks are, of what they are composed, and how they are 

 arranged, before attempting to explain these facts. In such a 

 study, however, we should meet with so much that would be quite 

 unintelligible, that a more convenient way will be to begin with 

 a study of the agencies which are at work upon and within the 

 earth, and which tend to modify it in one or other particular. In 

 other words, we must employ the present order of things as a key 

 by means of which to decipher the hieroglyphics of the past, and 

 proceed from what may be directly observed to past changes 

 which can only be inferred. 



We might assume that the present was so radically different 

 from the far- distant past, that the one could throw no light upon 

 the other. Such an assumption, however, would be most illogical, 

 for there is nothing to support it. There is no reason to imagine 

 that physical and chemical laws are different now from what they 

 have always been, and the more we study the earth, the more 

 clearly we perceive that its history is a continuous whole, deter- 

 mined by factors of the same sort as are now continuing to 

 modify it. Some geologists assume that these agencies have 



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