PEEFACE TO THE THIKD EDITION. 



The progress of American geology is so rapid — important new dis- 

 coveries follow one another in so quick succession — that any text-book, 

 however carefully prepared, must require large revision in a very few 

 years. In the present edition the alterations, by omission, by modifi- 

 cation, and especially by additions, are so numerous and so great that 

 it was found necessary to reset the whole work, and to rewrite a large 

 portion. I have tried to do this without enlarging to any considerable 

 extent the size of the book 



The most important changes are the following : In Part I I have 

 made some additions to the discussion of river-agencies, especially in 

 regard to the mutual relations of erosion and sedimentation ; and to 

 rivers as indicators of crust-movements. On the subject of earth- 

 quakes I have left out the general discussion of waves, as belonging 

 strictly to physics, and have given more fully the subject of seismom- 

 etry. On coral reefs I have given a brief account of the theory of 

 Murray on the formation of atolls and barriers. I have stricken out 

 entirely the section on Geographical Distribution of Organisms, as be- 

 longing either to Biology or to Physical Geography, and to make room 

 for more strictly geological matters pressing for recognition. But I 

 have made compensation for this by a much fuller discussion, in Part 

 III, of the geological causes of present distribution. 



In Part II the structure and position of stratified rocks are largely 

 rewritten, and some changes introduced. The discussion on mineral 

 veins has been somewhat enlarged, and many changes introduced in 

 the discussion of faults and their causes. The section on mountains 

 has been entirely rewritten, the order of presentation changed, and 

 new matter introduced. 



In Part III the changes are still more extensive, and the law of 



