Davis. — Significant Features of Reef -burdt red Coasts. 19 



far as I can learn, low spur-end bluffs fronted by narrow rock platforms are 

 of much more common occurrence than strong spur-end cliffs that plunge, 

 except for their fringing reefs, into comparatively deep water. But fortu- 

 nately certain islands possess strong spur-end cliffs fronting on deep lagoons : 

 such islands deserve particular attention. 



The HalJ-submerged Cliffs oj Tahiti.— Tahiti, the largest island of the 

 Society Group, is a volcanic doublet — that is, a larger and a smaller cone 

 connected by an isthmus — now submaturely dissected by radial consequent 

 valleys. In the central areas, where the valleys are close together and of 

 great depth, the initial surface of the cones is lost ; but it is well preserved 

 in the peripheral areas, where the valleys are more widely separated and of 

 moderate depth, as Dana long ago explained. The inter-valley spurs are, 

 except at the north-western (or leeward) corner of the large cone, cut back 

 in cliffs, which on the windward coasts rise 500 ft. to 1,000 ft. above present 

 sea-level. Agassiz is, as far as I have read, the only observer of 

 this beautiful island who has recognized the prevalence of cliff's around 

 its shores. 



Many of the smaller valleys are not cut clown to present sea-level ; their 

 wet-weather streaius fall in cascades from cliff'-toji notches. The larger 

 valleys have been cut to a greater depth, for they descend below sea-level, 

 and their mouths are occupied either by small arms of the sea or by delta- 

 plains. The island is to-day bordered either by a fringing reef or by an 

 alluvial plain, which is formed by the confluence of many delta-plains that 

 have outgrown their valley-mouth embayments. Moreover, a somewhat 

 discontinuous barrier reef now holds off" the waves from most of the island 

 circuit. Evidently, then, the cliffs have not been cut while the island has 

 stood at its present level : they must have been cut when it stood relatively 

 higher — that is, when the valleys were in process of deep erosion beneath 

 present sea-level. Evidently, also, no reefs could have been present when 

 the cliffs were cut. 



The question then arises whether Tahiti stood still and had its cliffs 

 cut while the ocean was lowered and chilled during the Glacial period, or 

 whether Tahiti, besides experiencing changes of ocean-level in the Glacial 

 period, itself subsided after cliff's had been cut around it, the cliff's having 

 been formerly cut around Tahiti for the same reason that cliffs are now 

 cut around Reunion — namely, because reef-forming corals cannot establish 

 their colonies on the cobbles and gravels of the beaches that are ordinarily 

 developed around the shore of a young volcanic island. 



The latter alternative appears the more probable one of the two, for 

 two reasons. First, the amount of the submergence by which the Tahitian 

 valleys have been submerged appears to be 500 ft. or 600 ft. at least, and 

 this is much more than the amount of lowering that the ocean is believed 

 to have suffered during the Glacial period. Secondly, if the cliffs of Tahiti 

 were cut around a still-standing island by the waves of the lowered and 

 chilled ocean during the Glacial period, then the neighbouring island of 

 Murea, as well as the other more distant members of the Society Group, 

 should also be cut back in cliff's ; but, apart from a few very exceptional 

 cliff'ed spur-ends, that is not the case. The reefs of Tahiti should therefore 

 be regarded not as having found their opportunity for upgrowth when the 

 warming waters of the post-Glacial ocean were rising to their present level, 

 but as having found their opportunity when submergence, caused in part at 

 least by subsidence, embayed the island valleys so that the stream-washed 

 detritus was pocketed in the embayments. In the absence of detritus the 



