1893.] evidence for the Recurrence of Ice Ages. 233 



the freezing-point is found at sea-level. Jn the British Isles it 

 just touches the tops of our highest mountains. In the Alps it 

 lies between 7000 and 9000 feet above sea-level. Exceptional 

 geographical conditions bring it down to sea-level in South-West 

 Greenland on the latitude of Southern Norway. 



It was quite possible that denudation might have proceeded 

 pari passu with upheaval in every case and no high ground be the 

 result of the long continued upheavals of which we have such 

 abundant evidence. It might have been that this was one of the 

 automatic compensations provided by nature and that no great 

 elevations were ever thus produced. But there they are — moun- 

 tains and table-lands — existing in the present and ever recurring 

 in the past. 



The upheaval is a fact ; what we have to ask is only whether 

 in any particular case the balance of uplifted land left after ages 

 of denudation is enough to explain the occurrence of glacial 

 conditions. If we find traces of glaciation we say — you have in 

 elevation an ordinary operation of nature sufficient to account for 

 it. If we find no evidence of excessive cold then we say — in 

 denudation is another well-known operation of nature which has 

 proved in this case sufficient to counteract the effect of the vast 

 upheaval. 



If the Scotch mountains, the tops of which are now about 

 freezing-point, were uplifted 3000 feet we should not only get a 

 fall of 10° to 12° in temperature due to the greater height, but the 

 enormous accumulation of snow and ice would still further lower 

 it and glaciers would protrude their cold mass far over the now 

 warm lowlands. An elevation of 30,000 feet with the greater 

 precipitation from adjacent warmer seas would go far to furnish 

 all that has yet been proved of tropical glaciation. 



Summary. 



I have endeavoured to get rid of some sources of error arising 

 out of a wrong reference of certain phenomena to glacial action 

 and want of due regard to the various modes of transport of 

 glaciated material, showing that there are so many ways in which 

 stones are accidentally striated that the greatest caution is neces- 

 sary with regard to the character and origin of the scratches 

 observed upon them, and that there are so many modes of trans- 

 port and imbedding of boulders that we require the clearest 

 evidence as to all the circumstances in which they are found. 



I explained that I had had exceptional opportunities of forming 

 an opinion as to the value of the evidence in the principal cases 

 relied upon in proof of the occurrence of glacial conditions in 

 the Palaeozoic and Mesozoic rocks of this country, and had come 



