PREFACE. 



IN choosing from a large number the following papers for 

 republication, it may be well to state the considerations which 

 have guided the author in his selection. His researches and 

 his conclusions as to the chemistry of the air, the waters, and 

 the earth in past and present times, the origin of limestones, 

 dolomites, and gypsums, of mineral waters, petroleum, and me- 

 talliferous deposits, the generation of silicated minerals, the 

 theory of mechanical and chemical sediments, and the origin 

 of crystalline rocks and vein-stones, including erupted rocks and 

 volcanic products, cover nearly all the more important points 

 in chemical geology. They have, moreover, been by him con- 

 nected with the hypothesis of a cooling globe, and with certain 

 views of geological dynamics, making together a complete 

 scheme of chemical and physical geology, the outlines of which 

 will be found embodied in essays I. - XIII. of the present 

 collection. It was at one time proposed to rewrite for this 

 volume the first seven of these, giving them a more connected 

 form, and thereby avoiding some little repetition ; but it is 

 thought better to reproduce them in the shape in which they 

 originally appeared, and this chiefly for the reason that they 

 seem to the author to have a certain historic value, and serve 

 to fix the dates of the origin and development of views, some 

 of which, after meeting for a time with neglect or with active 



