118 Considerations on determining 



Desnoyers, to demonstrate the difficulty of determining, by Mr. Lyell's me- 

 thod, the relative ages of tertiary deposits, and since he admits that many 

 objections had been urged against the practical application of the percen- 

 tage test, by a previous writer, surely he might have been expected to give 

 some further intimation of the nature of those objections, even were it merely 

 for the purpose of strengthening his own cause. In treating upon all mat- 

 ters of scientific enquiry, either in recording new facts, or enlarging upon 

 the ideas of others, it is usual for some reference to be made to the condi- 

 tion in which the particular subject under investigation, has been left by 

 preceding writers. The reasons, however, which have led M. Desnoyers, in 

 this instance, to deviate from a course so obviously consistent with just and 

 honorable feeling, are but too plainly apparent. The " objections " spoken 

 of by this geologist, but without any explanation as to their nature or bear- 

 ing, are nothing more or less than the very same which now appear in the 

 Bulletin of the Geological Society of France, in the form of original sug- 

 gestions, by M. Desnoyers ; although they have been transferred to the pa- 

 ges of that work, from communications which have appeared, at various 

 times, in the scientific periodicals of this country. We would remark also, 

 that M. Desnoyers' acquaintance, as a practical geologist, with tertiary de- 

 posits, does not appear to have furnished him with one single objection a- 

 gainstthe employment of the per-centage test; his sole ground for opposing 

 its general introduction into the science, resting upon the opposite views en- 

 tertained by different naturalists, respecting the identification of fossil with 

 recent species. The amount of originality contained in M. Desnoyers' re- 

 flections, and the degree of assurance he must have possessed, to exclaim 

 " Sij'ai appele specialement l'attention des geologues conchyliologistes sur 

 cette divergence de determinations," (p. 209), can only be duly estimated by 

 presenting one or two passages from the memoirs, the contents of which he 

 appears to have so freely consulted. 



In the Magazine of Natural History for 1836, p. 537, an abstract is 

 given of a paper, read at the previous meeting of the British Association, 

 by the editor of the present series of this journal ; the title being, — "On. 

 some fallacies involved in the results relating to the comparative age of tertia- 

 ry deposits, obtained from the application of the test recently introduced by 

 Mr. Lyell and M. Deshayes." 



The following passage will be found in this article, at p. 537. 



" During the author's investigation of the fossiliferous strata above the 

 London clay, in Suffolk and Norfolk, some facts have come under his obser- 

 vation, which appear to him to point out sources of error to a considerable 

 extent, in the application of the test recently proposed by M. Deshayes and 

 Mr. Lyell, and which is now so generally made use of in the classification 

 of tertiary formations. 



" The crag has been referred, by Mr. Lyell, to his older pliocene period, 

 on the authority of M. Deshayes, who identified, among the fossil Testacea 

 of that deposit, 40 per cent, with existing species. The correctness of this 

 result has been called in question by other eminent conchologists, particu- 

 larly 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 met with only two or three crag shells, which may, perhaps, be iden- 

 tified with existing 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 pre- 

 cisely similar result has attended Dr. Milne Edwards's examination of the 

 corals. 



"Professor Phillips, in his Introduction to Geology, has placed the crag 



