JALUIT. 285 



fig. 4). Large blocks of Porites and of Maaandrina, some of them fully five 

 feet in height, occur on the flats of the lagoon side ; they are the remnants 

 of the mass of beach rock conglomerate which was once elevated above the 

 present level of the summit of the reef rock conglomerate beach. It was 

 interesting to examine the rolled corals on the outer beach and to trace 

 the gradual disappearance of the typical characters of the different species 

 of corals, showing when fresh and first thrown up on the beach all their 

 specific characteristics, losing them as they become gradually worn by the 

 action of the sea, until finally the rolled masses are changed to rounded 

 pebbles, which have lost their coral features and look much like the 

 pebbles of any shingle beach. We made a large collection of the rolled 

 corals to show the different conditions under which the same species may 

 be found in close proximity, and to assist in determining the fossil corals of 

 some of the elevated coral reef ledges, where they must have been subjected, 

 before being fossilified, to the same modifying agencies as those of the 

 present day. 



The surface of the beach rock conglomerate or breccia on the outer 

 sea-face platform is full of potholes of all shapes and sizes (PI. 165, fig. 2), 

 many of them still partially filled with the pebbles to which they owe their 

 origin. While the sea is rushing up and down across the face of the outer 

 reef flat, the pebbles are revolved rapidly in the potholes, wearing them 

 deeper and deeper, until finally they reach a depth where the action of the 

 sea becomes less or ceases. Adjoining potholes are often gradually united, 

 according to their shape and position on the outer reef flat, forming deep 

 bowl-shaped recesses, or more or less narrow troughs and intricate channels. 



The large blocks found on the sea face of the reef flat (PI. 165, fig. 2) 

 have either been torn off from the knolls of the outer edge of the reef flat, 

 or else they are a part of the conglomerate beach rock or breccia ledge left, 

 owing to their greater hardness, after the erosion of the conglomerate on 

 the flat. The slope of the outer platform is quite flat (Pis. 164 ; 165, fig. 2 ; 

 167, fig. 2) ; a comparatively shallow sink extends between the outer knolls 

 and the base of the shingle beach. 



Outer reef platforms as they exist in the Pacific and Indian Ocean are 

 only slightly developed in the West Indies. 



