GLACIAL-CONTROL THEORY OF CORAL REEFS. 181 



is nearly 2 m. per annum. ^^ The average rates of annual recession 

 for the chalk cliffs of Normandy and for those of Dover are' said to be, 

 respectively, 0.3 m. and 1 m. Fischer found the rate for the hard 

 rocks of Algiers to be 10 m. in 1,200 years. ^^ 



The average rock-strength of most of the Pleistocene islands and 

 coastal plains was doubtless no greater, and probably less, than that 

 of the Dover cliffs, which also have height of the same order as the 

 quantity d. On the other hand, the energy of the waves annually 

 breaking on the English cliffs is less than that of the waves annually 

 breaking on an equal length of coast-line in the oceanic islands. ^° 



If, as is probable, the tropical ocean was more stormy in the Glacial 

 period than it is now, the rate of cliff recession was correspondingly 

 higher. 



The peneplanation of an oceanic island, initially as large and lofty 

 as Hawaii, would produce a composite of volcanic and shelf material 

 about as extensive as the Macclesfield bank, one of the very larg- 

 est coral platforms known (a "submerged atoll"). Such a mass 

 would be attacked on all sides by the strong Pleistocene waves. 

 The pre-Glacial embankment of sand, mud, and organic debris would 

 yield at least as fast as the clayey cliffs of Yorkshire are receding before 

 the relatively small waves of the North Sea. Probably 2 m. per 

 annum would be the minimum rate of recession for cliffs developed in 

 these shelf deposits. If the lava flows of the central mass were deeply 

 weathered, the rate of cliff' recession there might a\erage 0.5 m. or 

 more per annum. 



As already observed, the wave abrasion began before the climax 

 of the Kansan stage and continued without serious interruption until 

 the Wisconsin climax — a period estimated as 280,000 to 900,000 

 years. Is it too extreme to hold that, during such a long period, the 

 surf of the open ocean, sweeping in on all sides, would abrade every 

 part of an island even so extensive? Is it too extreme to believe that 



28 E. R. Matthews, Coast Erosion and Protection, London, pp. 11, 21, 22 

 (1913). 



29 See E. Briickner, in AUgemeine Erdkunde (Hann, Hochstetter. and Pok- 

 orny), Prag and Wien, Abt. 2, p. 260 (1897). 



30 This statemnit holds true in spite of the fact that wind waves are specially 

 aided by the tides in their attack on the cliffs of England. C. Darwin (The 

 Structure and Distribution of Coral Islands, London, 3rd ed., p. 86 (1889)) 

 remarked that, if the corals of any one of the many low coral islands were 

 killed, the whole island "would be washed away and destroyed in less than 

 half a century.", On page 129 of his book he notes that a single storm entirely 

 truncated two of the Caroline islands and partly destroyed two others. Manj'^ 

 similar cases are on record. 



