274 MAPS ILLUSTRATING REVOLUTIONS CHAP. xiv. 



being united. This he called the second elephantine period ; 

 and it would coincide very closely with that part of the post- 

 pliocene era in which man coexisted with the mammoth, and 

 when, according to Mr. Trimmer's hypothesis, the Thames 

 was a tributary of the Rhine.* 



These geographical speculations were indulged in ten years 

 after Edward Forbes had published his bold generalisations 

 on the geological changes which accompanied the successive 

 establishment of the Scandinavian, Grermanic, and other living 

 floras and faunas in the British Islands, and, like the theories 

 of his predecessor, were the results of much reflection on a 

 vast body of geological facts. It is by repeated efforts of 

 this kind, made by geologists who are prepared for the partial 

 failure of some of their first attempts, that we shall ultimately 

 arrive at a knowledge of the long series of geographical 

 revolutions which have followed each other since the begin 

 ning of the post-pliocene period. 



The map, fig. 39, p. 276, will give some idea of the great 

 extent of land which would be submerged, were we to infer, 

 as many geologists have done, from the joint evidence of 

 marine shells, erratics, glacial striae and stratified drift at 

 great heights, that Scotland was, during part of the glacial 

 period, 2,000 feet below its present level, and other parts of 

 the British Isles, 1,300 feet. A subsidence to this amount 

 can be demonstrated in the case of North Wales by marine 

 shells (see above, p. 267). In the lake district of Cumberland 

 and Yorkshire," and in Ireland, we must depend on proofs 

 derived from glacial striae and the transportation of erratics 

 for so much of the supposed submergence as exceeds 600 

 feet. As to central England, or the country north of the 

 Thames and Bristol Channel, marine shells of the glacial 

 period sometimes reach as high as 600 and 700 feet, and 

 erratics still higher, as we have seen above (p. 270). But 



* Joshua Trimmer, Quarterly Geological Journal, vol. ix. plate xiii. 1853. 



