68 A. J. Jukes-Browne — Ghalk-2)ebbles on tlie 



material to 35, and the fact of their being so riddled with the cavities 

 made by boring animals points to their having consisted of a less hard 

 kind of chalk, and is consistent with their attribution to the ordinary 

 tough chalk of the cortentudmanum zone. 



From the preceding descriptions and comparisons it will be seen 

 that the pebbles of hard chalk which have been obtained by the 

 Marine Biological Association from the floor of the English Channel 

 seem to be referable to hard beds in the zones of Jl. Cuvieri, 

 Holaster planus, and Micraster cortestudinarium. This is really what 

 might have been expected from a consideration of the component beds 

 of the Chalk in Devon and Dorset, the most persistent hard beds in 

 these counties being found in the zones above-mentioned. 



It is not surprising that nothing like the Melbourn Rock can be 

 recognized among the pebbles, because in its typical aspect it does not 

 reach so far west as Devonshire; neither ought we to expect anything 

 exactly like the Beer Stone to occur among them, for that is entirely 

 a local development even in Devonshire, as it only occurs between 

 Beer and Branscombe on the coast and again at Sutton Barton, near 

 Offwell, 6 miles to the north. Hence the Beer Stone must have been 

 accumulated as a bank of shell-sand little more than a mile in width, 

 but possibly 8 or 10 miles long from north to south. Lastly, it is not 

 surprising that chalk formed 60 or 70 miles farther south should 

 possess some peculiar and special features of its own. 



The identification of these chalk-pebbles with beds occurring in the 

 zones above-mentioned raises some intei'esting questions. If they 

 have been derived from the destruction of chalk in situ we may infer 

 that the Turonian and Senonian deposits extended southwards across 

 the whole of South Devon and the adjacent Channel area as far south 

 as lat. 49'50, which is about that of Valognes, in Normandy. We 

 have then to ask whether the Cenomanian and Selbornian deposits 

 which underlie the Turonian in Devonshire are likely to have had 

 a similar extension. 



Mr. Worth has assumed that the deposits from which the pebbles 

 were derived did occur within the area in question, and I think he is 

 justified in the assumption, but it may be well to give reasons for 

 this belief. Such a varied assortment of pebbles, if transported to 

 their present position, could only have been carried there in one of 

 two ways; either by rivers flowing over a land surface or by 

 floating ice. 



These pebbles lie on the northern slope of the Channel floor, not 

 in a depression which might be regarded as a portion of a submerged 

 river valley (see Map, p. 63). The only large river which is likely to 

 have bi'ought them would be a continuation of the Tamar and Tavy, 

 and such a river may have existed in Eocene times, or might in 

 Miocene time have obtained them from Eocene gravels. Now ancient 

 gravels which are possibly of Eocene age do occur on Cattedown, 

 near Plymouth, and have yielded a piece of Lias limestone but no 

 chalk-pebbles ; neither do such pebbles occur in the other Tertiary 

 gravels of Devonshire, so that this source is improbable. 



That they should have been transported by floating ice during the 

 Glacial epoch seems still more unlikely, for chalk-pebbles could not 



