428 NATURAL SCIENCE. august 



detached episodes. Yet, as with casual finds of ancient documents, 

 a study of these episodes will often throw light on what happened 

 much later, though the intermediate history may still be a blank. 



The first question here to be dealt with will be, What was the 

 temperature in Britain during the Glacial Epoch, and how far did 

 the cold affect more southern regions ? In discussing this matter, 

 I do not propose in any way to speculate on the cause of the climatic 

 change ; under the present circumstances it is, perhaps, better to 

 treat the question simply as one of evidence. There are several 

 theories in the field ; if one of them can be made to fit the facts, 

 well and good, but we must not allow any preconceived theory to 

 bias us in this enquiry. 



When we attempt to learn what climatic conditions held during 

 past times, several different classes of evidence are available. We 

 can treat the question as a purely physical one ; we can treat it from 

 a botanical point of view ; or from a zoological. Finally, we can 

 combine the independent chains of evidence, and if they all lead to 

 the same conclusion we may consider that we have established a 

 safe working hypothesis. 



To the naturalist the most interesting part of the enquiry will 

 probably be. What was the lowest annual temperature met with 

 during the Glacial Epoch ? This question, till lately, would have 

 been difficult to answer with any confidence, but recent advances in 

 our knowledge of the present condition of the Arctic Regions, and 

 recent studies in Pleistocene geology, make a fairly accurate estimate 

 quite possible. 



The temperature of the sea and of the air do not necessarily 

 correspond in the same regions. We will, therefore, first discuss 

 the evidence as to the lowest temperature of the seas around Britain. 

 For this purpose the former southern limit of floating ice, and the 

 southern limit of the formation of shore ice, or " ice-foot," ought to 

 give a fairly accurate idea as to the temperature of the water. No 

 doubt a large iceberg may travel a long distance through compara- 

 tively warm water before it entirely melts away ; but shore ice, such 

 as forms every winter in the Arctic Regions, must float in water 

 nearly as cold as itself if it is to carry its burden of beach-stones and 

 erratic blocks any great distance. 



The English Channel, during the greatest severity of the Glacial 

 Epoch, seems to have occupied the southern limit of the formation of 

 ice-foot — or, at any rate, of shore-ice sufficiently thick to transport 

 anything but small stones. Ice-foot such as collects every winter in 

 the Arctic Regions then fringed our south coast, and beset the shores of 

 Brittany and of the Channel Islands. When, in the spring, this ice 

 became detached it transported its burden of included rocks hither 

 and thither, even across the Channel. We thus find at Selsea Bill, 

 in Sussex, erratics weighing several tons, but undoubtedly transported 

 from Bognor or from the Isle of Wight. Others, equally large, have 



