478 PKOCEEDINGS OF THE GEOLOGICAL SOCIEir. 



Crag. Of the first great change produced by the emergence of land 

 to the south previous to the formation of the Red Crag we have 

 scanty evidence. Traces of a Crag of the age probably either of our 

 Coralline Crag or of the Crag noir exists in Touraine; so that until 

 that period there had been communication with southern seas and 

 an interchange of species*. The elevation of the Wealden dome 

 brought to the surface beds of early Crag or of Diestien age, portions 

 of which still exist in our North Downs at an elevation of from 500 

 to 600 feet t ; and a like elevated tract, capped by beds of the same age, 

 is prolonged into France and Belgium. This formed a barrier sepa- 

 rating the southern and northern sea areas, and so isolating the 

 fauna of the Coralline Crag, that, with the lower temperature of the 

 sea resulting from the exclusion of currents from the south and the 

 setting in of others from the north, a great part of that fauna died 

 out. It is a case of extinction by change of conditions, and not by 

 time. 



This accounts for the disappearance in the Red Crag, noticed long 

 since by Mr. Searles Wood, of so large a number of the southern 

 genera of shells which characterize the Coralline Crag ; while the 

 descent of the more northern genera continues, with little loss, ac- 

 companied by the introduction from time to time of new species of 

 northern forms. 



A considerable number of the species which disappeared from our 

 area at the period of the Crag continued to exist further south in 

 the Atlantic and Mediterranean. Other species, under favourable 

 conditions of the low temperature at great depths, survived in the 

 mid- Atlantic, where their existence remained unknown until they 

 were recovered by the deep-sea dredging so successfully carried on 

 of late years. As many as 93 species of the Coralline Crag have 

 been found at greater or less depths in the southern seas ; and of 

 these, 17 met with at depths of from 1000 to 7000 feet had not 

 been before met vdth living J. In the same way there are 65 species 

 of the Red and Norwich Crags now found ranging to great depths ; 

 but of these, 39 lived at the time of the Coralline Crag ; and of 4 of 

 these not before known, 2 are Coralline-Crag species. 



The presence of northern and arctic species does not, however, 

 necessitate a severe climate ; for cold currents may give a northern 

 facies to the sea-fauna, while the land may retain the mean tempe- 

 rature due to geographical position. Beyond the introduction of 

 more northern forms of shells in the Red and Norwich Crags, there 

 is nothing to indicate a great increase of cold. None of the blocks 

 of the Coralline Crag have been found drifted far from the Sutton 

 islet. The porphyry boulder at the base of the Coralline Crag ex- 

 ceeds in dimensions any other foreign block either in the Red or the 

 Norwich Crag. In the Chillesford Clay itself nothing but large 



* Only a few shells of the Faluns of Touraine passed into our Crag area ; but 

 many of the Bryozoa are common to the Coralline Crag and Douay beds, 

 t Quart. Journ. Geol. Soc. vol. xiv. pp. 322 et seqq. 

 I Quart. Journ. Geol, Soc. Tol. xxvii. p. liv. 



