4 "ALBATEOSS" TUOriCAL PACIFIC EXPEDITION. 



tan, in Ua pn Island, also at Hanaiaj^a, Hiva Oa Island, at Anaho Bay 

 on the north side of Nnkuhiva, and in Comptroller Bay on the south 

 coast, besides at several points in Tai-o-hae Bay. 



I can only attribute the absence of coral reefs in the group to the 

 fact that there have been no great platforms of erosion formed at the 

 base of the slopes of these islands. It is difficult to imagine why such 

 platforms do not exist here, for tlie angle of the slopes and their char- 

 acter are not different from that of other volcanic groups where both 

 fringing and barrier coral reefs occur. Owing to the greater depths 

 near the coast line, corals find a depth suitable for their growth only 

 in the closest proximity to land ; there is no opportunity for lateral 

 expansion, as in volcanic islands which are flanked by a wide platform 

 of erosion, on which the soundings are comparatively shallow or far 

 within the depth at which corals can grow. The absence of this plat- 

 form of erosion does not mean that these volcanic islands are not 

 eroded fully iis fast as similar islands in Fiji and in the Society group 

 in about the same latitude, and under similar conditions of exposure 

 to the tradewinds. A glance at the charts' will show how extensive 

 this erosion has been. The steep bluffs or the sloping faces, which 

 in one case flank the inner side of the reef platform, in the case of 

 the Marquesas simply flank the edge of the shore, but if they slough 

 off, they become disintegrated and worn into volcanic sand or small 

 masses on which coi'als can find no footing, as is also the case in the 

 Galapagos. This seems to me to be the probable explanation of the 

 absence of corals in the Marquesas ; it is confirmed by what exists in 

 the Galapagos^ and in the island of Mehetia to the southeast of Tahiti, 

 where the absence of a shore platform is most striking, the more so as 

 Mehetia is in a region where corals are wonderfully developed upon the 

 shore platforms of other volcanic islands in the Society Islands. 



Round the base of the extinct crater of Mehetia there are neither 

 barrier nor frino-ino: reefs. Corals are limited to a small area at the 



1 A. Chart 1640. 



^ In the Galapagos district the absence of corals is partly due to other causes. Bull. M. C. Z., 

 XXIII., No. 1, p. 69. 



