Daly — The C oral-Reef Zone. 1 5 1 



the heavy sculpturing or accretional formation had been 

 accomplished long- before. 14 



3. Evidence from Cliff ed Islands. — If reefs have not 

 been successfully planted on a pre-Grlacial volcanic island, 

 its lavas suffered abrasion in pre-Glacial time as 

 well as during the whole Glacial period. Lacking reef 

 protection during the interglacial stages, the island has 

 probably had an erosion history different from that of 

 most islands in the coral-reef zone. Partially drowned 

 Pleistocene cliffs, modified by Recent erosion, might be 

 expected. Tahiti, Tutuila, Reunion, and Hawaii seem to 

 be examples of such exceptional islands. Why are they 

 exceptions ? 



The delay in finding reef protection is conceivably due 

 to one or more of several causes : 



a. Situation of the island near the border of the tem- 

 perature zone where corals can flourish, rendering suc- 

 cessful reef growth impossible because of intermittent 

 chilling of the shore region by shifts of currents. 



o. Failure of colonization by corals, owing to the fact 

 that prevailing currents have not been charged with coral 

 larvae. 



c. Too great abundance of detritus on the island 

 shores, tending to smother coral plantations — a principle 

 specially emphasized by A. Agassiz in his description of 

 the Marquesas islands, the Galapagos group, Mehetia, 

 etc. (See Memoirs, Mus. Comp. Zool., vol. 28, p. 4, 1903.) 

 Several conditions make for turbidity of the shore water. 

 If the island is largely composed of volcanic ash and 

 agglomerate, both subaerial and marine erosion unite in 

 increasing the supply of shore detritus faster than if the 

 island were entirely composed of lava flows. 15 Other 

 things being equal, great altitude for the island tends to 

 speed up the delivery of stream-borne detritus to the 

 shore belt. Other things being equal, the larger the 

 island the more rapidly is land waste supplied to the 

 shore belt, because the gathering ground for the waste 

 from an island of constant shape increases as the square 



14 The Glacial-control theory by no means implies that the great Maccles- 

 field Bank is the root of a volcanic island first truncated by Pleistocene 

 waves (as stated by W. M. Davis, Bull. Geol. Soc. America, vol. 29, p. 520, 

 1918). 



15 Hence one must study the petrography of each island before drawing 

 conclusions as to its fitness to support reefs. 



