BIGELOW. — SHOAL- WATER DEPOSITS OP THE BERMUDA BANKS. 589 



W. M. Davis that the oscillations can probably be detected to a depth of 

 several thousand feet. Moreover, Alexander Agassiz ('88", p. 273) writes 

 that off the New England coast disturbing forces due to the action of 

 waves, tides, and currents may extend perhaps to a depth of nearly three 

 hundred fathoms. But in this case currents are probably chiefly concerned. 

 More definite and satisfactory are the opinions of Admiral AVhartou ('97), 

 who has suggested that the depth to which the action of waves extends 

 may be indicated by the change of slope generally taking place off shore 

 below a depth of eighty to one hundred fathoms ; and, further, that the 

 existence of banks in the open sea at a depth of from thirty to forty 

 fathoms may show the limit of depth to which oceanic waves may cut 

 down a land mass on which they act. Agassiz > (lOS", p. xviii.) also thinks 

 there is good evidence of such submarine denudation. At first glance 

 the Challenger Bank seems to present a good example of such a worn- 

 down bank ; dredging has shown, however, that its surface is not under- 

 going a process of denudation, but rather one of growth. 



If we can accept Admiral Wharton's conclusions, — and they rest upon 

 a much sounder basis than the earlier views which limited wave action 

 to lesser depths, — we shall find in wave action, and especially in the 

 oscillations of storm waves, a sufficient explanation for the present 

 problem. I am convinced that the frequent movements of the nuUipore 

 concretions, which undoubtedly occur, are brought about in this way ; 

 and I consider the occurrence of such spherical concretions on the bank 

 good evidence that at a depth of from thirty to fifty fathoms the action of 

 storm waves is often of considerable force. 



The dredgings from the Challenger Bank add to the evidence already 

 accumulated to prove the great importance of nullipores as reef builders. 

 The active role which they play in the economy of coral reefs, especially 

 on the sea faces, and in localities where the corals are dead or dying, 

 has been emphasized by Agassiz ('88% p. 82), who, writing of the Florida 

 reefs, says, " Immense masses of nullipores and corallines grow on the 

 shallowest flats at the tops of the branches of madrepores which have 

 died from exposure to the air." J. Stanley Gardiner ('98, pp. 501, 502), 

 iu his studies on the island of Funafuti, also comes to the conclusion that 

 they take a very important part in the building up of a coral reef. They 

 are not, however, limited to shallow waters, but occur at considerable 

 depths, being absolutely limited only by the absence of sunlight. Thus 

 Agassiz ('88) found them smoothing over the modern limestones on the 

 Pourtales Plateau, in depths of from ninety to three hundred fathoms, 

 and in the borings at Funafuti nullipores seem to be the most important 



