S. V. Wood,jun. — American and Britkh Surf ace- Geology. 27 



destroying agency of the ice-sheet to which the Upper Glacial clay 

 of East Anglia was due, and to be more inclined than he was to adopt 

 the view of the later age generally of the Scotch beds ; but he is not 

 definite upon the subject, and still seems to regard the Scotch Till 

 as including beds of all stages of the Grlacial period. I am glad, 

 however, to see that he is more disposed (as, for instance, in the case 

 of the Wexford gravels') than he was to allow molluscan evidence to 

 have its weight in determining the age of Glacial deposits. 



Mr. Geikie has thought it necessary to put a special erratum at 

 the commencement of his second edition, that he finds Mr. Jeffreys 

 stating in Phillips' Geology of Yorkshire, that instead of there being 

 five of the Bridlington shells not known as living, all of them are 

 identical with species found living in Arctic and Northern Seas. It 

 should be known, therefore, that these identifications are not matters 

 of recent discovery, such as from dredgings or otherwise, but matters 

 of opinion upon which Mr. Jeffreys is not in accord with some of 

 his fellow conchologists. That difference of opinion extends to a 

 considerable number of the Pliocene mollusca, Mr. Jeffreys' method 

 being to call any Pliocene shell which comes near to a species known 

 living as identical with it, or at most a variety of it ; but if after- 

 wards something more closely resembling the fossil is discovered 

 recent, then the former identification is abandoned, and the living 

 species first called identical with or a variety of the fossil is sepa- 

 rated from it, and the fossil form is allowed to stand as a good 

 species, because it is found living. By pursuing through the Miocene 

 and Eocene fauna, successively, this method of calling all Pliocene 

 shells that approximate to living ones varieties of the latter, and 

 these again varieties of their Miocene and Eocene predecessors, some 

 astonishing results may be elaborated in the way of making a large 

 per-centage of Eocene Mollusca identical with living species, con- 

 trary to the generally received opinion of geologists. 



So slight, however, is the change which has taken place in shells 

 during long periods, that many Eocene shells do not differ more 

 from their nearest living analogues than do certain of these Bridling- 

 ton ones ; and if Mr. Jeffreys can point out any clear natural rule or 

 law which will define where varietal difference ends, and specific 

 difference begins, he will throw great light upon the study of living 

 and fossil organisms, and cause evolutionists seriously to re-consider 

 their views. 



Two out of the five thus said by Mr. Jeffreys to be living are Nucula 

 Cohboldice and Tellina ohliqua. These shells abound together in the 

 newer beds of the Crag, and in the East Anglian Glacial. Both are 

 found at Bridlington ; and one, the Nucula, in the centre of the 200 

 vertical feet of morainic clay of Dimlington in Holderness,^ in such 



1 The "Wexford grayels, if the fauna given from them be reliable, would also seem 

 to be a remnant of the beds of the earlier glaciation in Ireland ; for along with some 

 other peculiar forms, a fi-agment of Nucula Cobholdice is mentioned by Forbes as 

 having been found in them. It is, however, possible that this fragment might have 

 been one of Lucina [Loripes) divaricata, a living British shell. 



^ The place is indicated on the accompanying map, where also the dotted line shows 

 the position of the Chalk floor down to which, from contiguous borings, the chalky 

 clay has been found to descend. 



