PREFACE. 



and they are chiefly therefore interesting as they 

 are connected with the mineral history of rocks. 

 But the different species of the latter, are, in 

 many cases, intimately connected with the views 

 that may be entertained respecting the different 

 condition of the globe at distant periods. Thus 

 they form an interesting part of the geological 

 history of rocks ; sometimes, indeed, offering the 

 only proofs that can be obtained of similarity or 

 difference of character, in cases where the geo- 

 logical analogies or differences are of an import- 

 ant nature. 



To have given therefore the names of these 

 organic species in connection with the rocks 

 which contain them, and of which the mineral 

 characters alone are described in the following 

 work, would have added materially to its value; 

 more particularly as it is intended to facilitate 

 the study of geological relations. This however 

 was impossible. Our information on this subject 

 is as yet so limited, that many species remain not 

 only und escribed and unnamed, but scarcely any 

 admit of being certainly referred to a constant 



