XARRATIVK OF TIIK KXPEDTTTOX IX ls9G. 11 



The soinidiugs showed a coral liottom at great depths round the atoll, not uncom- 

 monly between 500 and 800 fathoms, and in one instance, the most northern sounding- 

 marked on the chart, sand and coral are shown at a depth of 1354 fathoms. This 

 spot is situated a little over 3^- sea miles from the nearest part of the reef 



The belt of land Avhich separates the ocean from the lagoon presents much the 

 same character as in the case of Keeling Atoll, described by DarwIxV. A glance at 

 the two diagrams (fig. 4) is sufficient to show this. 



The "outer edge" of Darwin's diagram (1 of fig. 4) corresponds to our " nullipore 

 rim," which owes its deep madder-red tint to one group of these algse. Forming a 

 low mound which rises to a height of about one foot above low tide, at the extreme 

 seaward margin of the land, the nullipore rim is visible for the greater part of 

 the day ; and by its rich colour produces a pictorial effect so fortunate as to seem 

 placed there almost by design ; it is the last note of colour offered by the land, and 

 pleases both by its contrast with the whiteness of the surf, and by its perfect 

 harmony with the deep blues of sky and sea. Its algal layers extend on to the 

 platform behind, and on breaking away fragments of this with a hammer, it is 

 found to be tunnelled through with numerous close-set tubes and other burrows, 

 which harbour a rich variety of life. 



Behind the rim lies a rocky platform, Darwin's " flat of coral rock " (C of 

 fig, 4) ; this is covered with a few inches of water next the rim, even at low 

 tide, and its middle portion is frequently somewhat higher than either its seaward 

 or landward sides, and so becomes exposed during the ebb as a broad, low swell of 

 rock bounded on either hand by shallow channels, and supporting numerous pools 

 upon its surface. (See Plate A, upper figure, where A is the nullipore rim ; B, the 

 shallow depression in the reef flat ; C, the reef flat.) 



The reef-flat passes into a gently sloping glacis, which rises landwards to about 

 ordinary high-water level. (Plate A, lower figure, where the outer line of waves 

 marks the nullipore rim ; the reef flat is submerged, the glacis is seen rising from the 

 sea, ending landwards in a miniature scarp, succeeded by loose sand, &c. ; palms 

 crowming the outer ridge, here rather lower than usual.). 



Both flat and glacis consist of hard limestone, smoothed by the waves for the most 

 part, but becoming rougher nearer the landward end. It is with respect to this that 

 Darwin remarks :"'- — ■" I could with difficulty, and only by the aid of a chisel, procure 

 chips of rock from its surface, and therefore could not ascertain how much of it is 

 formed by the aggregation of detritus, and how much by the outward growth of 

 mounds of corals, similar to those now living on the margin," and I fancy the once 

 prevailing impression that a coral-reef would be found on boring to consist of hard 

 limestone, partly owed its origin to the universal presence of this rock on the 

 seaward face of the atoll betw^een tides. Our boring operations, especially in our 

 second attempt, showed that the limestone is of no great thickness, and that uncon- 



* ' Coral Eeefs,' p. 15 (Ed. 3). 

 C 2 



