STRUCTURE OF CORAL ISLANDS. 65 
lands. The encircling atoll-reef, corresponds with the outer reefs that 
enclose high islands; and the green islets with the beach formations, 
in the two cases, originate in the same manner. 
The lagoons, moreover, are similar in character and position to the 
inner channels within barrier reefs; they receive only coral material 
from the action of degrading agents, because no other source of 
detritus but the reefs isat hand. The accumulations going on within 
them are, therefore, wholly of coral. The reefs within the lagoons, 
correspond very exactly in mode of growth and other characters to 
the énner reefs under the lee of a barrier. The corals grow but little 
disturbed by the waves, and the reef-rock thus formed, often contains 
them in their natural positions. 
The preceding descriptions, represent the general character of atolls, 
but are more especially drawn from the Paumotus. There are some 
peculiarities in other seas, to which we may briefly allude. 
Among the scattered coral islands north of the Samoan Group, the 
shore platform is seldom as extensive as at the Paumotus. It rarely 
exceeds fifty yards in width, and is cut up by passages often reaching 
almost to the beach. It was not unusual for our boats to obtain a 
landing by watching for a favourable opportunity at the entrance of 
one of these channels to mount a wave and ride in on its top. In some 
places the platform is broken into islets. Einderby’s Island is one of 
the number to which this description apples: the beach is eleven or 
twelve feet high. For the first eight feet, it slopes very regularly at 
an angle of 30 to 35 degrees, and consists of sand, coarse pebbles, or 
rounded stones of coral, with some shells; and there is the usual beach 
conglomerate near the water’s edge. After this first slope, it is hori- 
zontal for eighty to two hundred feet, and then there is a gradual 
rise of three to four feet. Over this portion there are large slabs of 
the beach conglomerate, along with masses from the reef-rock, and 
some thick plates of a huge foliaceous Madrepora; and these slabs, 
many of which are six feet square, lie inclining quite regularly against 
one another, as if they had been taken up and laid there by hand. 
They incline in the same direction with the slope of the beach. The 
large Madrepora alluded to has the mode of growth of the Madrepora 
palmata; and probably the entire zoophyte extended over an area 
twelve or fifteen feet in diameter. ‘The fragments are three to four 
inches thick, and thirty square feet in surface. 
As a key to the explanation of the peculiarities here observed, it 
17 
