Daly— The Coral-Reef Zone. 149 



be mentioned here, since they have been thought to invali- 

 date the theory of Glacial controls. 18 



1. Evidence from Browned Valleys. — During post- 

 Pliocene time the earth's crust, in the tropical belt, was 

 not perfectly stable. Some areas in the coral-reef zone 

 have sunk; others have risen. Warpings, illustrating 

 both kinds of movement in one and the same crustal block, 

 are practically demonstrated. Certain volcanic islands 

 have been formed since the Pliocene. Many other vol- 

 canic islands have undergone deep trenching by streams. 

 Doubtless some of the younger volcanic islands have 

 recently sunk, for the special reasons already noted. 

 Certain embayments seen in volcanic islands, as well as 

 exceptional depths in lagoons, such as 70 fathoms at 

 Vanikoro, may possibly be thus explained. 



Yet the proof of recent local movements of the kind is 

 far from being a proof of subsidence as a general cause 

 of atolls and barrier reefs. Much more weight should be 

 given to the highly specialized relations existing between 

 reefs on the one hand and shelves and detached banks on 

 the other. 



The objection to the Glacial-control theory, based on 

 the existence of drowned valleys, loses force when it is 

 recognized that the Tertiary and later subsidence, so 

 demonstrated for some central islands and for certain 

 continental coasts, is not necessarily connected with the 

 upgrowth of living reefs. As previously explained, 

 embayments due to pre-Glacial, local subsidence may 

 have lost much of their detrital filling through Pleisto- 

 cene erosion and thus may resemble embayments caused 

 by quite recent drowning of valleys. That the drowning 

 in several instances is to be dated well before the dawn 

 of the Glacial period is proved by the width of the shelves 

 fronting those bays. Many other embayments have size 

 and form appropriate if they have been caused by a rise 

 of sea-level no greater than that taking place since maxi- 

 mum glaciation. In fact, the latter group seems to 

 testify to the probable soundness of an important element 

 in the Glacial-control theory of reefs, namely, the tempo- 

 rary but prolonged lowering of base-level of tropical 

 streams. 



13 See especiallv W. M. Davis, Journal of Geology, vol. 26, pp. 198-222, 

 289-309. 385-411, 1918: and Bull. Geol. Soc. America, vol. 29, pp. 489-574, 

 with bibliography, 1918. 



