Geological Society. 295 



than Mr. Webster that in Headdon Hill (which gave the types of 

 all his formations above the London clay), the middle argillaceous 

 group contains innumerable freshwater shells, greatly predominating 

 over the marine, and bands of lacustrine marl differing in no respect 

 from that of the upper and lower groups that in Norton Cliff (about 

 two miles north of Headdori Hill), the three groups are mineralogically 

 well developed without containing a single marine fossil that at 

 Hampstead Cliff, where the argillaceous marls have four or five times 

 their average thickness, no undoubted marine shells appear on the 

 true parallel of the upper marine formation* and that in many other 

 parts of the Isle of Wight the three groups admit not either of mine- 

 ralogical or zoological separation from each other ; but are composed, 

 from top to bottom, of an indefinite number of alternations of argilla- 

 ceous and calcareous marls, passing at one extremity into soft unc- 

 tuous clay, and at the other into freshwater limestone f. 



Facts like these prove, if I mistake not, the impossibility of insti- 

 tuting any rigid comparison between all the successive groups in the 

 basins of Paris and the Isle of Wight. But discrepancies in minute 

 details militate in no respect against Mr. Webster's leading gene- 

 ralizations, which have received such a striking and unlooked-for 

 illustration in the fossil mammalia of Binstead. If the hints now 

 thrown out should induce him to lay before the public some part of 

 his valuable observations on our different tertiary deposits, or to 

 hasten the publication of his long-promised work on the Isle of 

 Wight, my present purpose will be completely answered. 



In the papers a brief analysis of which I have now placed before 

 you, we have some new and striking proofs of the great importance 

 of organic remains in determining the comparative age of remote 

 and discontinuous formations. And we have seen that in cases where 

 we have few examples of specific agreement, we can, from the aspect 

 of large groups of fossils and the general resemblance of their generic 

 types, form at least a probable estimate of the age of the deposits to 

 which they are subordinate. Inferences of this kind would be alto- 

 gether worthless were they invalidated by the direct evidence of 

 geological sections. But we deny that this is in any respect the case j 

 and our conclusions are the more certain, because they are not only 

 founded upon a wide induction of particulars, but are consistent 

 among themselves. 



There can be no doubt that in the ancient ocean, as well as in the 

 present, the distribution of organized beings was affected by many 

 causes by the temperature and depth of the waters by the nature 

 of the soundings by the action of tidal currents and by other unap- 



* In the highest part of the argillaceous marls of Hampstead Cliff (about 

 two miles east of Yarmouth), there are, however, two species of Corbulae; 

 but they occur, if I mistake not, far above the parallel of the "upper marine 

 marls" of Headdon Hill. 



f Anomalies, similar to those pointed out above, are stated also to occur 

 in portions of the Paris basin, and may perhaps hereafter be used as terms 

 of comparison with the structure of the Isle of Wight. 



preciable 



