XVI PREFACE. 



will be found described by the numerous authors 

 who have written or compiled treatises on this 

 subject. Here, it has been sufficient to enu- 

 merate them under the titles of the rocks in 

 which they occur. 



But, independently of these minerals, of che- 

 mical composition, numerous organized bodies, 

 the remains of animals or vegetables, existing at 

 remote periods during different states of the 

 earth's surface, are found imbedded in rocks. 

 These also, like chemical fossils, have distinct 

 characters, dependent on their organization, 

 which are of an interesting nature, but which, 

 like those of minerals, form objects for a separate 

 treatise, connected with the departments of botany 

 and zoology. 



One important difference however exists be- 

 tween chemical and organized minerals, as to 

 the value which they severally possess in a geolo- 

 gical view. From the former, little comparative 

 information is derived respecting the different 

 states of the earth's surface at different periods ; 



