XXV111 PREFACE. 



form. On comparing also these details with the 

 observations of others, where the secould be divest- 

 ed of those theoretical opinions which sometimes 

 obscure them, he has found no reason to doubt of 

 their truth. If he has not referred to those 

 authors, abundant reasons might be shewn for the 

 omission, some of which must be obvious to every 

 one acquainted with the subject, but into which 

 he cannot here enter without endangering a 

 lengthened detail and invidious discussions. He 

 believes that the facts which are here stated are 

 generally in harmony with those that are to be 

 found in the works of his predecessors and cotem- 

 poraries who have seriously and practically inves- 

 tigated the same subject, with the wish to learn, 

 and with that preliminary knowledge without 

 which no one can see. As to those who have 

 written, without observing, it is not in geolo- 

 gical science only, that it is necessary to overlook 

 the " questionable reporters of the questionable 

 authority of others." 



With respect to some of the geological pre- 

 faces, they may appear of an undue length. 



