1891. J D. Prain — The Vegetation of the Coco Group, 291 



as the defence seems, this shingle beach completely prevents erosion 

 though at the same time accretion is probably very slow. In the case 

 of the highest and most advanced coral reefs usually the same shingle 

 beach occurs ; from which fact we might conclude that as the initial 

 stage of any fringing-reef must have been that of ia simple submerged 

 sandstone ledge of greater or less extent, we see here the original shingle 

 beach, thrown np where this ledge originally became subaerial, to which 

 the waves have eaten back over the present raised reef until all the 

 sandy soil formed during the earlier " embankment and pool " stage has, 

 with the vegetation it supported, been swept into the sea. This shingle 

 having been reached the erosive action has been checked, and the surer, 

 if slower process of shingle accumulation has been initiated or, at all 

 events, renewed. From this account of these bays it will be seen that 

 the fringing-reef exhibits in some parts a phase more advanced than it 

 exhibits in others. But it does not therefore follow that these more 

 advanced " platform " portions are older than the earlier " embankment 

 and pool " portions. They cannot, in one sense, be so old, for we must 

 suppose that all these reefs commenced contemporaneously, and the 

 " embankment and pool " reefs are still growing, whereas the " platform " 

 reefs have now no living coral. The different stages therefore merely 

 indicate that the sandstone reefs running out from the headlands in 

 which the various ridges end are in different parts of the islands situated 

 at different depths, and the condition of the reefs indicates that the 

 sandstone ledges are shallower, and that deep water is further from the 

 shore towards the south than towards the north end of the islands. At 

 quite the southern extremity of Little Coco bare sandstone reefs, too 

 shallow for the growth of a coral fringing-reef, stretch away south- 

 eastward in much the same way as the well-known Alguada reefs extend 

 southward off Cape Negrais. On the east coast of Little Coco are high 

 coral reefs exposed at low-tide, fringed by a coral-shingle beach, while 

 towards the north end of the island are similar high reefs fringed by a 

 shore of sandy soil which, with the beach-forest growing on it, is being 

 washed away by the sea. On the west coast, where the reefs ai'e high, 

 and, though still in the " pool " stage appear from their jagged edges 

 to be approaching the " platform " stage, a line of low sand-dunes, per- 

 haps the highest development of the epoch of sand-accretion, have been 

 thrown up ; these at present protect the shore and have actually closed 

 up, at the south-west corner, the mouth of a mangrove-creek. 



Similai'ly, in Great Coco, near the southern extremity and between 

 the main island and Jerry there is a large bare sandstone reef which ex- 

 hibits very well the arrangement and dip of the strata ; further up the 

 east coast denudation is going on, still further north the site of a beach- 

 38 



