182 DALY. 



a relatively smooth surface of al)rasion was completed just below the 

 last low, Pleistocene sea-level? To achieve that end the waves of the 

 whole Glacial period cooperated, but the final smoothing was accom- 

 plished at the last Glacial climax. The abrasion was, of course, 

 accompanied by a new slight increase in the width of the encircling 

 embankment. 



Whatever doubt may exist as to the ability of the Pleistocene waves 

 to develop so large a platform, there can be little as to their power to 

 truncate completely the average volcanic island which had been 

 peneplained and deeply decayed before the Glacial period. In this 

 case the width was less than 20 km., and about one-half of the area 

 abraded was composed of weak shelf deposits. Still more clearly 

 would the truncation be accomplished if the central volcanic mass had 

 been already once truncated, by pre-Glacial waves. (See Fig. 4.) 



In striking contrast were such islands as Hawaii, Tahiti, Murea, 

 or Rotuma, the charts of which show submarine benches so narrow as 

 to prove the extraordinary power of fresh lavas to resist the Pleisto- 

 cene breakers. 



The benching of the pre-Glacial continental shelves was in general 

 the work of waves running in only from two quadrants, instead of four, 

 as in the case of the oceanic island. On the other hand, these shelves 

 were not usually composed of any other material than weak sediments, 

 irregularly veneered with tougher masses of reef coral. Cliff recession 

 should therefore be rapid, and some of the wave-cut benches might be 

 expected to have widths measuring in the tens of kilometers. 



Depth of the Pleistocene Benches Below Present Sea-level. In weak 

 materials open-ocean waves can quickly form a bench about 10 m. 

 below low-water level, but abrasion at greater depths is indefinitely 

 slower. In the course of 50,000 years the depth of the bench surface 

 would, probably as a rule, not be increased to more than 20 m., though 

 its outer part might be at depths of 30 m. to 40 m. If the maximum 

 lowering of level in the tropical ocean during the Pleistocene brought 

 it 55 m. (30 fathoms) below present sea-level, the bench surfaces then 

 cut would not be deeper than 75 m. to 95 m. (40 to 52 fathoms) below 

 the same datum. If the Pleistocene sea-level within the tropics were 

 75 m. lower than the present one, the benches might locally have the 

 depth of 115 m. (63 fathoms). The corresponding minimum depth is 

 naturally zero. Between these limits are to be found the facets cut by 

 the Pleistocene waves. 



The facets cannot be perfectly smooth and level, nor even in the 

 case of those due to the complete truncation of islands, are they all at 



