230 DALY. 



Consider the case of a late-Tertiary volcanic island, of normal type, 

 composed of massive flows, with some interbedded layers of ash. 

 Before the shore became well protected by growing corals, it was 

 subject to some cliffing, a narrow bench being cut in the lavas. Out- 

 side the rock bench was a narrow shelf constituted of the detritus 

 washed from the land by streams and waves. On this composite 

 terrace the corals settled, and, with the development of a fringing reef, 

 the waves were no longer able to attack the island successfully. The 

 killing of the corals in the Glacial period caused a resumption of wave- 

 benching. The sea-level being then 50 m. to 75 m. or more lower 

 than before the ice-caps were formed, the waves quickly benched the 

 outer part of the terrace but soon discovered the hard lava underlying 

 the terrace detritus. Thenceforth cliff recession must have been not 

 only slow but increasingly slower, since the cliff grew higher as the 

 line of surf advanced into the gently sloping volcanic foundation. 

 Assuming the sea-level to have been then constantly 55 m. below its 

 present position, the height of the sea-cliff would have to be more than 

 55 m., if any of that cliff should be now visible. The recession of the 

 cliff to the point where it was 55 m. high might well occupy most or 

 all of the time during which the sea-level had nearly its maximum 

 depression. If the volcanic spur was cliffed to a greater height, post- 

 Glacial weathering and washing might have much softened its crest, a 

 few meters above present sea-level. 



Of course, the greater part of each Pleistocene sea-cliff, now below 

 sea-level, is buried by post-Glacial detritus and reef material. Such 

 a buried cliff, more than 20 m. high, has been demonstrated by 

 borings through the coral reef and underlying mud at Brandy Bay, 

 Sumatra. The cliff rock there is andesitic.^^ 



The Tertiary sea-clift's of the imagined island were subject to wast- 

 ing through all the time following the original development of coral 

 reefs on the island; that is, from the late Tertiary, or earlier, to the 

 present day. Those cliffs would be expected to have been much 

 softened in contour, if not wholly extinguished as distinct facets, before 

 the present epoch. 



In general, the volcanic islands exist because they are composed of 

 rocks that are relatively resistant to the weather and to wave abrasion. 

 Their existence, as well as the usual absence of very high spur-cliffs, 

 merely shows that the Glacial period was of limited duration. On 

 the other hand, atolls give no direct evidence as to its duration. If 



62 C. p. Sluiter, Petermann's Geog. Mitt., 1891, Lit. Ber., p. 46. 



