THE PACIFIC OCEAN. 273 
these animal secretions must have been deposited 
within that distance from the surface. At the same 
time, it is no less true that the water in the immediate 
vicinity of the islands is fathomless, and that the 
descent of their outer edge is remarkably abrupt 
and precipitous. The only.satisfactory explanation of 
the phenomenon appears to be the one proposed and 
ably supported by Mr. Darwin, in his elaborate 
treatise on Coral reefs. Many islands of the com- 
mon rock formation are found in the Pacific, on the 
shelving sides of which, a few fathoms below water, 
the coral animals have fixed their stony habitations, 
forming what is called a fringing reef, distinguished 
from others by being immediately attached to the 
land, without the intervention of any lagoon or 
channel of water. Mr. Darwin supposes that every 
island in the Pacific originally presented this struc- 
ture, but that wherever a variation at present exists, 
the solid rock has been gradually, and ‘perhaps very 
slowly, subsiding to a lower level. Now, let us 
assume this state of things for a moment, and look at 
the results. We must, however, mention two well- 
ascertained instincts of the Polype: the one is, that 
it works up towards the light; the other, that its 
proceedings are most vigorous at the outer edge, 
where it is washed by the beating waves. Let A 
represent the section of a rocky island; B, B, the level 
of low-water; and D, the reef of coral fringing the 
coast. After the lapse of time, during which it has 
been subsiding, the water-level stands at 4, 5; the 
coral at D has died from the too great depth, but the 
animals have been working upwards upon the dead 
18 
