part 1] ARCTIC FLOEA OF THE CAM YALLEY. 21 



Discussioisr. 



Prof. J. E. Maee congratulated the Author upon the valuable 

 addition which she had made to our knowledge of the Pleistocene 

 geology of Cambridge. He called attention to the occurrence of 

 unworn implements of Upper Palaeolithic age on a terrace some- 

 what north of Barnwell, and at a height less than that of the 

 plant-bearing deposits. This proved that the plant-deposits were 

 formed prior to the end of Upper Palaeolithic times. The 

 occurrence of Zannichellia in these beds was by him unexpected, 

 and probably complicated the question as to various earth-move- 

 ments in later Pleistocene times. 



Mrs. E. M. Reid said that the Author's w^ork was most 

 valuable. She agreed with the Author that the botanical 

 evidence threw no light on the absolute age of the Arctic floras of 

 the Cam and Lea Valleys, but that it did throw light on their 

 relative ages, and showed that these were not identical. She had 

 been led to this view by comparing the two Arctic floras with the 

 Temperate floras which preceded (Cromerian) and succeeded 

 (present day) the Glacial Period. The two Arctic floras, as known, 

 contain almost the same number of species ; but, whichever com- 

 parison is made, the Lea-Valle}^ flora is seen to contain about half 

 as many again of Temperate forms as the Cam- Valley flora. This 

 shows that the Cam- Valley flora lived nearer to the maximum of 

 cold than the Lea- Valley flora, but does not show whether the Lea- 

 Valle}^ flora was earlier or later. Of the 145 known Cromerian 

 species, only 16 are found in the Cam- Valley flora, a deflnite indica- 

 tion that the Cromerian flora was mostly exterminated in Southern 

 Britain, and must have survived outside our islands. A great deal 

 is now known of these Arctic floras. With the return of warmth 

 the}^ were driven to higher latitudes and higher altitudes ; some 

 species reached both, some only one or the other, and some neither : 

 the latter were exterminated. In the speaker's view, plant life has 

 been driven to and fro, and up and dow^n the mountains, by stress 

 of climate. If we can but follow its migrations, we shall have a 

 most valuable botanical time-record, by which to trace changes of 

 climate. If such a record is ever made (and the speaker saw no 

 reason why in the future it should not be made), it would be by 

 the aid of such reliable and valuable work as that done by the 

 Author. 



Prof. W. J. SoLLAS expressed his satisfaction in learning that 

 Prof. Marr's discovery of the Dryas Bed at Cambridge had led 

 to such valuable results, and congratulated the Author on a 

 remarkable contribution to our knowledge of the Pleistocene flora. 

 We owe to Dr. and Mrs. Clement Reid an ancillary branch of 

 investigation which is bearing excellent fruit, and it is most 

 fortunate that a Cambridge botanist has appeared to continue its 

 cultivation. 



It was to be hoped that the Dryas flora is confined to a single 

 horizon. Whether it really was so or not might be still an open 



