1893.] evidence for the Recurrence of Ice Ages. 99 



of different length but of regular recurrence and should always 

 have a definite relation to the poles. Whereas, assuming the 

 geographical view to be the more true we should expect to find less 

 regularity as to the intervals, but still some secularity, and a con- 

 stant relation to the areas and axes of greatest earth movement. 



"We have therefore to search the record of the rocks to see 

 whether each great period has its glacial epoch represented in 

 circumpolar regions, and, if not, then before we can accept the 

 astronomical theory we must explain the absence of glacial 

 phenomena by supposing that that particular Geological period 

 was too short to cover the time required for the recurrence of 

 cold-producing combinations or we must grant that glacial action 

 has never extended so far or that all traces of it have been 

 obliterated over the area open to examination. 



We may fairly expect that some traces should remain ; for, 

 although terminal moraines on land might stand little chance of 

 surviving a period of submergence when each portion of the old 

 land was successively brought within the action of the encroaching 

 waves, still we must remember that a large part of the glacial drift 

 at the present day finds its resting-place far out to sea, whither 

 it is transported, if not by the land ice- foot at any rate by bergs 

 and other forms of sea-borne ice. 



If we fail to find distinct evidence of the recurrence of glacial 

 action in the rocks of circumpolar regions then we should examine 

 the known areas of successive upheaval in equatorial and tem- 

 perate, as well as in arctic or subarctic regions, in order to see 

 whether we can find there proofs of elevation sufiicient to give 

 rise to glacier ice and can find evidence in the ancient deposits 

 that glacial conditions did there prevail. 



In order to be in a position to discuss this question it will be 

 well first to consider the nature of the evidence upon which we 

 have to rely for the establishment of the fact of glaciation, and I 

 have thought that the best way of bringing this matter before the 

 Society was to exhibit samples of the specimens which have been 

 appealed to in support of special views of glaciation and certain 

 other specimens, of which we know the vicissitudes and the mode 

 of production, in illustration of the characters relied upon. 



ii. Ghar^acters of glacial deposits. 



Two principal classes of phenomena have to be studied in con- 

 nection with ice-borne drift. First the condition of the included 

 fragments, and secondly the arrangement of the material. Many 

 of the inferences with regard to the mode of transport and distri- 

 bution of the material are founded on the condition of the included 

 boulders, and the most important of the characters impressed upon 

 the rocks by glacial agencies are the polished and grooved surfaces 



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