Vol. 54.] AND THE CORALLIN-E CRAG. 845 



seem equally improbable. The Belgian geologists know of no such 

 invasion of their country by the sea. They are rather of opinion 

 that it receded in a northerly direction towards the end of the 

 Diestien period (see map, fig. 1, p. 316). Moreover, such a sub- 

 sidence, greatly increasing the depth of the North Sea and carrying 

 it over a largely extended area, must have brought about an entirely 

 new set of conditions, with new shore-lines, an altered system of 

 drainage, and sediments as well as currents of a different character. 

 Alterations in the relative level of land and sea are supposed to 

 take place very slowly, and a depression and re- elevation of such 

 magnitude would probably have been attended not only by the 

 deposition of beds varying in composition and commensurable in 

 importance with the protracted period which they represented, but 

 also by some marked changes iu the molluscan fauna. The Pliocene 

 strata of Holland, which are many hundreds of feet in thickness, 

 and have accumulated pari passu with the subsidence which has 

 affected that country, are, for example, easily separated into zones, 

 even by the chance specimens found in boring. It seems incon- 

 ceivable, therefore, that if such vast changes had taken place in t?ie 

 conditions of the Crag basin as those postulated by Prestwich, 

 the Crag fauna would have remained substantially unaltered during 

 the whole time, and that the various stages would have been severally 

 represented by a few feet only of current-caused sediment of the 

 same character throughout, even when tested by chemical analysis. 

 I am not aware that a single fact has been adduced, either by the 

 geologists of England or the Continent, in confirmation of the theory 

 of a great subsidence during the Coralline Crag epoch. 



I am equally unable to accept Prestwich's view that at one 

 part of the Coralline Crag period the temperature of Northern 

 Europe fell so considerably as to permit of the presence of floating 

 ice in the Crag basin, bringing boulders into it either from Scan- 

 dinavia or the Ardennes. This hypothesis, which seems to rest on 

 the otherwise unexplained presence of a single waterworn block 

 of porphyry (' neither angular nor striated ') ^ in the basement-bed 

 at Sutton, seems to me at variance both with the fossil evidence and 

 with the probabilities of the case. 



Among the extinct species of the Coralline Crag we find, as is 

 well known, a number of genera characteristic of warmer seas than 

 our own, while the recent forms are preponderatingly southern. 

 With one exception, Buccinopsis Dalei,^ a survivor from Miocene 

 times, but not at present known living south of the western coast 

 of Ireland, all the more abundant recent mollusca of the Coral- 

 line Crag are now to be found, either in the Mediterranean, or along 

 the Atlantic coasts of France and Portugal, while a third of the 

 number have an exclusively southern range.^ Purely northern 



^ Quart. Journ. Geol. Soc. vol. xxvii (1871) p. 117. 

 2 See also Geol. Mag. 1896, p. 27. 



' There is a similar absence of boreal and arctic shells from the Diestien beds 

 of Belgium. 



