FAKAEAVA. 79 



Nullipore knolls are found ujasses of Pocillipores, which here, as elsewhere 

 in the Tropical Pacific, seem to flourish remarkably well in such an exposed 

 habitat as the outermost edge of the reef platform. 



On the lagoon side in many places a low sand beach, at the outside three 

 to four feet in height, has been thrown up, inside of which runs a long trench 

 or depression, the bottom of which is at about the level of the water in the 

 lagoon at half tide, and is more or less full of brackish or fresh water accord- 

 ing to the season of the year (PI. 48). Similar trenches or sinks filled with 

 water are found in many of the Paumotus ; they vary greatly in shape 

 according to their position. If occurring along the lagoon face of a narrow 

 island, they are mere trenches, but may become sinks or pools or even small 

 ponds of considerable size, if they have been formed by the isolation of a 

 wider tract of low land, such as at the extremity of an island near a pass, 

 where the land usually widens out. Such pools have been described at 

 Rangiroa (PI. 204), on the south of Avatoru Pass, and are known in many 

 other atolls in the Paumotus, in the Ellice and Marshall Islands, which are 

 described as having ponds or sinks filled with more or less brackish water.' 

 Dana has described them as occurring at (Manihi) Waterlandt, so named 

 from the presence of a number of such trenches and pools or sinks. One of 

 the sinks to the south of the village of Rotoava must be fully four feet deep 

 and at least 200 feet long by 30 wide.^ These sinks, when of considerable 

 length and width or depth, have been described as secondary or subsidiary 

 lagoons. I shall have occasion to describe their formation in other atolls 

 where their structure is more easily seen than in Fakarava, as in the lagoons 

 of Likieb and Jaluit. 



The effect which the violent surf has in modifying the mode of growth of 

 corals is very striking, and it would often be difficult to recognize in the 

 spreading mass found on the outer reef flat the delicately branching 

 species characteristic of the lagoon. Everything living on the sea face 



The long sinks more or less parallel to the shore of the island are formed hv the throwing up of 

 a low beach sand dam, behind which water collerts. At one of the points we really have an atoll 

 within an atoll, formed by the ring of islets and sand bars cutting across the shallow reef flats of the 

 point. This we shall find developed on a larger scale in other atolls not only of the Paumotus, but also 

 in the Gilbert and IMarshall Islands. 



'^ Dana (p. 324) mentions a sink in Aratika 50 feet in diameter. 



