Davis. — Sir/nificaiif Features of Beef-bordered Coasts. 



21 



The Vanished Detritus of Deeply Denuded Islands. 



Many volcanic islands, now deeply denuded in irregular forms, give clear 

 indication of their initial conical form in the outward slant of their 

 marginal lava-beds. It is in such cases a comparatively simple matter to 

 reconstruct their original cone, VW (fig. 5), and to estimate the volume 

 of detritus that has been removed in reducing the island to its present 

 maturely denuded form, EM. Even if no submergence be assumed, tlie 

 volume of detritus that has been carried away from so much of the initial 

 volcanic mass as is now above sea-level is, as noted above, vastly greater 

 than the volume of the lagoon waters, Gr, on all the reef-encircled islands 

 that I have seen. How has this great volume of detritus been disposed of ? 



Fig. 3. 



Let the island be supposed to have been formerly more emerged than 

 now, and let it stand still with respect to sea-level, SC, during a period of 

 deep dissection. Under these conditions the detritus washed out frrm its 

 valleys would soon completely overwhelm any fringing reef that might by 

 chance be established on its shores, and the waves would then cut cliffs, 

 CL, all around its circuit, as is now the case on Reunion. This considera- 

 tion alone is sufficient to discredit Murray's theory of outgrowing reefs on 

 still-standing islands. Moreover, if the island stand still, cliff-cutting will 

 continue and no opportunity for barrier-reef formation will be allowed. 

 Under Avhat conditions, then, is the formation of barrier reefs permitted ? 



An apparent escape from the difficulty of accounting for the vanished 

 detritus around a still-standing island is found in changes of ocean-level 

 during the Glacial period ; for the detritus discharged while the ocean stood 

 at a lower level than now would be deposited on the lower slopes of the 

 island, and when the ocean rose again a barrier reef might grow up with it. 

 But during the discharge of the detritus reefs could not flourish, and waves 

 would then cut the island-shores back in cliffs ; and if cliff -cutting endured 

 through the time required to excavate the valleys now drowned in embay- 

 ments the cliffs would surely be high enough to be still visible after the 

 ocean has resumed its normal level. Hence the amount of submergence 

 thus provided is insufficient for the needs of the problem. Moreover, all 

 volcanic islands the eruptional growth of which was completed earlier than 

 the beginning of the Glacial period should have had cliffs cut around their 

 margin in pre-Glacial time, and some trace of these cliffs should now be 

 found. Another supposition must therefore be made, as follows : — 



If an island, VW (fig. 6), with sea-level originally at NV, does not stand 

 still, it must subside to a great depth, NS, if no cliffs are to be cut around 



