144 PROFESSOR JAMES GEIKIE ON THE 



iii the succession. The Forest-bed, one might suppose, indicated an interglacial epoch, 

 separating two cold epochs. But Mr Clement Eeid, who has worked out the geology of 

 the Pliocene with admirable skill,* has another explanation of the phenomena. It has 

 lonsf been known that the organic remains of the marine Pliocene of Britain denote a 

 progressive lowering of temperature. The lower member of the system is crowded with 

 southern forms, which indicate warm-temperate conditions. But when we leave the 

 Older and pass upwards into the Newer Pliocene those southern forms progressively 

 disappear, while at the same time immigrants from the north increase in numbers, until 

 eventually, in the beds immediately underlying the Forest-bed, the fauna presents a 

 thoroughly Arctic facies. During the formation of the Older Pliocene with its southern 

 fauna our area was considerably submerged, so that the German Ocean had then a much 

 wider communication with the seas of lower latitudes. At the beginning of Newer 

 Pliocene times, however, the land emerged to some extent, and all connection between 

 the German Ocean and more southern seas was cut off. When at last the " Forest-bed 

 series " began to be accumulated, the southern half of the North Sea basin had become 

 dry land, and was traversed by the Ehine in its course towards the north, the Forest- 

 bed representing the alluvial and estuarine deposits of that river. 



Mr Eeid, in referring to the progressive change indicated by the Pliocene marine 

 fauna, is inclined to agree with Professor Peestwich that this was not altogether the 

 result of a general climatic change. He thinks the successive dying out of southern 

 forms and the continuous arrival of boreal species was principally due to the North Sea 

 remaining fully open to the north, while all connection with southern seas was cut off. 

 Under such conditions, he says, " there was a constant supply of Arctic species brought 

 by every tide or storm, while at the same time the southern forms had to hold their own 

 without any aid from without ; and if one was exterminated it could not be replaced." 

 Doubtless the isolation of the North Sea must have hastened the extermination of the 

 southern forms, but the change could not have been wholly due to such local causes. 

 Similar, if less strongly-marked, changes characterise the marine Pliocene of the Medi- 

 terranean area, while the freshwater alluvia of France, &c, furnish evidence in the same 

 direction. 



The Cromer Forest-bed overlies the Weybourn Crag, the marine fauna of which 

 has a distinctly Arctic facies. The two cannot, therefore, be exactly contemporaneous : 

 the marine equivalents of the Forest-bed are not represented. But Mr Eeid points out 

 that several Arctic marine shells of the Weybourn Crag occur also in the Forest-bed, 

 while certain southern freshwater and terrestrial shells common in the latter are met with 

 likewise in the former, commingled with the prevailing Arctic marine species. He thinks, 

 therefore, that we may fairly conclude that the two faunas occupied adjacent areas. 

 One can hardly accept this conclusion without reserve. It is difficult to believe that a 

 temperate flora and mammalian fauna like that of the Forest-bed clothed and peopled 

 Eastern England when the adjacent sea was occupied by Arctic molluscs, &c. Surely 



* Mem. of Geol. Survey, " Pliocene Deposits of Britain." 



