of Tertiary Deposits. Ill 



with the existing species. The correctness of this result has 

 been called in question by other eminent conchologists, parti- 

 cularly by Dr Beck of Copenhagen, who has examined the crag 

 fossils in the author's collection, and considers that the whole of 

 them are extinct. In this opinion Dr Beck is supported by 

 Mr G. B. Sowerby, who states, that he has only met with two 

 or three crag shells, which may, perhaps, be identified with exist- 

 ing species. Professor Agassiz has inspected an extensive series 

 of ichthyological remains, collected from the crag by the author, 

 and pronounces them all to belong to extinct genera or species ; 

 while a precisely similar result has attended Dr Milne Edward's 

 examination of the corals. 



Professor Phillips, in his Introduction to Geology, has placed 

 the crag in the miocene or the middle tertiary division ; while Dr 

 Fleming, who, for more than a quarter of a century, has been an 

 indefatigable collector of British shells, considers that the propor- 

 tion of recent species in the fossils of that formation has been 

 rather under than over-rated by Deshayes ; and among the corals 

 of the crag he has detected a large porportion of living forms. 



The particular one of Mr LyelPs divisions, to which a geolo- 

 gist will refer any given deposits, must therefore depend upon 

 his own estimate of the characters which constitute specific dis- 

 tinctions, and which is evidently liable to the greatest possible 

 amount of variation. The author next enters upon an inquiry 

 respecting the course which should be adopted in obtaining the 

 relations of analogy, presented by the fossils of different depo- 

 sits to one another, or to the races in existence at the present pe- 

 riod. The effect of the method now made use of is, to class as 

 contemporaneous those deposits which respectively furnish the 

 same per-centage of extinct forms, without the slightest reference 

 to the greater or less degrees of approximation which these forms 

 exhibit, when compared with living types. The conchologists 

 who agree with Dr Beck cannot, by means of the per-centage 

 test, express the difference in the amount of approximation pre- 

 sented by the testacea of the crag and London clay, to those now 

 existing, because they would consider all the fossils of both these 

 formations extinct, and, consequently, refer them both to the 

 eocene or oldest tertiary division. 



In this instance, the relation of analogy can only be obtained 



