1893.] evidence for the Recurrence of Ice Ages. 223 



without losing their striae, but where the boulder is surrounded 

 by cla}^ packed up in the gnarled roots of a tree, or covered by 

 marine growths, it may travel far without being exposed to any 

 wear and tear that would obliterate the ice-marks. 



In the observations just recorded I have endeavoured to 

 explain the preservation of the grooves and scratches, but if we 

 are seeking only an explanation of the presence of a block far 

 from its source of origin and in material of very different coarse- 

 ness the explanation is easier. Stones travel steadily along a 

 coast with every tide and wind-directed shore current and it is a 

 matter of everyday occurrence to find a large boulder or two 

 appear and disappear in a few tides along a sandy or gravelly 

 beach. Although there is a definite proportion between the 

 velocity uf a current and the coarseness of the material which it 

 will move, this does not apply to a single large stone resting on 

 sand or small gravel. A boulder 2 feet in diameter would not 

 be moved from its place among boulders all of somewhat the 

 same size by a current which would scour out the sand around it 

 if it lay isolated on a sandy beach and would trundle it along the 

 shore — leaving it at last perhaps buried in clay in some depth 

 out of reach of the wind-waves and currents. 



These are only a few examples of the many modes of transport 

 of boulders. We must also remember that they have often been 

 derived from older boulder-bearing rocks and that they may even 

 have come originally from rocks now covered by newer deposits as 

 many of the boulders in the Cretaceous beds of East Anglia have 

 probably been derived from the subterranean plateau now becoming 

 so well known by the numerous borings for water for the supply 

 of London. 



With all these doubts as to the glacial origin of the striations 

 on rocks and stones, with so many modes of transport of boulders 

 and drift, with the abundant proofs of depressions of sufficient 

 extent to bring down to sea level the moraines and other traces 

 of glaciers which once occupied the mountain tops, we may be 

 pardoned for not accepting as proved all the reports of extreme 

 glaciation having occurred in equatorial regions with geographical 

 conditions the same as now. A theory involving such difficulties 

 as to have driven some of its advocates in desperation even to 

 shift the axis of the earth back and fore as required so as to bring 

 a portion of the equatorial region within reach of circumpolar 

 glaciation. 



