6 Mr. Charlesworth on the Crag, and on ascertaining 



ence, and not be supposed to invalidate the general course of 

 induction pursued by Mr. Lyell. Nevertheless it must be ad- 

 mitted that the practical application of the principle advocated 

 by this eminent geologist in the classification of the supracre- 

 taceous rocks will be extremely limited in operation ; for even 

 if we suppose that conchologists universally admit the sound- 

 ness of the principles upon which the present system of chro- 

 nological arrangement is founded, they cannot equally make 

 use of it as a means of obtaining numerical relations of affinity, 

 since the characters thought by one to constitute a distinction 

 of species are by another looked upon as mere modifications 

 of form. 



Now, if we entirely throw aside all reference to a per-cent' 

 age of species, and could substitute in its place a scale of de- 

 grees, — still taking the existing forms as a standard to which 

 the fossil ones are to be referred, but determining the amount 

 of approximation by the totality of the characters which each 

 series exhibits, — we might then, perhaps, justly anticipate 

 an agreement in the conclusions arrived at by different con- 

 chologists as to the relative age which should be assigned to 

 any one fossiliferous deposit of the tertiary group ; provided, 

 of course, that there be no difference in their respective quali- 

 fications for conducting the necessary examination*. 



Although Dr. Beck asserts that the Testacea of the crag 

 have no existing analogues, thereby necessarily placing that 

 formation in the eocene division of Mr. Lyell, yet as he ad- 

 mits a very considerable degree of resemblance in many of 

 these fossils to species now living in the German Ocean, I ap- 

 prehend that from that circumstance he would refer this de- 

 posit to a much more recent geological aera than the London 

 clay. If I am right in this conjecture, it follows as a neces- 

 sary consequence that there are two modes by which we esti- 

 mate the degrees of affinity between fossils of separate de- 

 posits, or between fossil and recent species, one being the 

 per-centage test, and the other that which must of necessity be 

 employed by Dr. Beck were he to infer the greater antiquity 

 of the organic remains of the London clay when compared with 

 those of the crag. 



Now it can be clearly proved that one of these modes is 

 sometimes fallacious, as there are tertiary deposits to which 

 if both tests be applied results completely at variance with 



ich other will be evolved. Thus many of the forms occur- 



tng in the coralline crag are so unlike recent types, that if our 



estima^of its comparative age were taken from the totality of 



* I am proceeding here upon the supposition that there is an uniform 

 approximation to existing species, shown by the fossils of different deposits, 

 correspondin^to their respective antiquity. 



igio th 



