MODE OF ORIGIN. 113 
and building up the reef, and in this work the two sides may go on 
with almost equal rate of progress: consequently we may often find 
no very great difference in the mzdth of the leeward and windward 
reefs, especially as the wind for some parts of the year has a course 
opposite to its usual direction. But seldom, except on the side to 
windward, is a sufficient force brought to bear upon the edge of 
the platform, to detach and uplift the larger coral blocks. The 
distance to which the waves may roll on without becoming too much 
weakened for the transportation of the uptorn blocks, will determine 
the outline of the forming land. With proper data as to the force of 
the waves, the tides, and the soundings around, the extent of the shore 
platform might be made a subject of calculation. 
The effect of a windward reef in diminishing the force of the sea is 
sometimes shown in the influence of one island on another. A striking 
instance of this is presented by the northernmost of the Tarawan 
Islands. All the islands of this group are well wooded to windward— 
the side fronting east, between north and south. But the north side 
of 'Tari-tari is nothing but a bare reef, through a distance of twenty 
miles, although the southeast reef is a continuous line of verdure. 
The smal] island of Makin, just north of Tari-tari, (see plate, page 
50,) is the breakwater which has protected the reef referred to from 
the heavier seas. 
Coral island accumulations have one advantage over all other shore 
deposits, owing to the ready agglutination of calcareous grains, as ex- 
plained on a following page. It has been stated that coral sand-rocks 
are forming along the beaches, while the reef-rock is consolidating in 
the water. A defence of rock against encroachment is thus produced, 
and is in continual progress. Moreover, the structure built amid the 
waves will necessarily have the form and condition best fitted for 
withstanding their action. The little islet of an atoll is therefore more 
enduring than those of harder basaltic rocks. Reefs of zoophyte 
growth but “mock the leaping billows,” while other lands of the same 
height would gradually yield to the assaults of the ocean. ‘There 
are cases, however, of wear from the sea, owing to some change of 
condition in the island, or in the currents about it, in consequence of 
which parts once built up are again carried off. Moreover, those 
devastating seas which overleap the whole land may occasion unusual 
degradation from some parts. Yet these islets have within themselves 
the source of their own repair, and are secure from all serious injury. 
29 
