PREFACE. 



Part of the present treatise was written originally in the 

 form of a supplement to my former work, entitled " Princi- 

 ples of Geology," and was intended for the use of those 

 students who found certain chapters in the Principles obscure 

 and difficult, for want of preliminary information. I after- 

 wards considered that it would not be incompatible with this 

 object to enlarge the Elements into a separate and independent 

 treatise, to serve as an introduction to Geology proper. As 

 I have thus been led on to become the author of two general 

 works on the same science, it may be useful to explain to 

 the reader that these two publications do, in fact, occupy very 

 distinct ground. 



In the Principles a systematic account is given of the ope- 

 rations of inorganic causes, such as rivers, springs, tides, 

 currents, volcanos, and earthquakes ; the effects of all being 

 particularly jonsidered, with a view to illustrate geological 

 phenomena. The changes also which the organic world has 

 undergone in modern times, the geographical distribution of 

 different species of animals and plants, the causes of their 

 multiplication and extinction, and their first introduction, are 

 discussed, and the various ways in which their remains 

 become fossil in new deposits. The student, who is familiar 

 with this the larger portion of the Principles (comprising no 

 less than five-sixths of the whole), will, it is hoped, more 

 easily comprehend the explanations of geological appearances 

 proposed in the Elements. On the other hand, those who 

 begin with the Elements, the scope of which may be under- 

 stood by a glance at the annexed table of contents, will follow 

 more easily the meaning of that part of the Principles in 



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