36 CORAL FORMATIONS. 
a line or two, the growing polyps of the surface having progressively 
died at their lower or inner extremity as they increased outward. 
The large domes of Astreas, which have been stated to attain some- 
times a diameter of ten or twenty feet, and are alive over the whole 
surface, owing to a symmetrical and unlimited mode of budding, are 
nothing but lifeless coral throughout the interior. Could the living 
portion be separated, it would form a hemispherical shell of polyps, 
in most species about half an inch thick. In some Porites of the same 
size, the whole mass is lifeless, excepting the exterior for a sixth of 
an inch in depth. ; 
~ With such a mode of increase, there is no necessary limit to the 
growth of zoophytes. The rising column may grow upward indefi- 
nitely, until it nears the surface of the sea, when death ensues simply 
from exposure, and not from any failure in its powers of life. The 
huge domes may enlarge till the same exposure just mentioned causes 
the death of the summit, and leaves only the sides to grow, which may 
increase indefinitely. Moreover, it is evident that if the land support- 
ing the growing coral were very gradually sinking, the upward increase 
of the coral might still be without limit. 
There is hence sufficient means provided for the production of coral 
material for islands, however numerous. ‘hese humble ministers of 
creative power might, without other attributes than those they now 
possess, have even laid the foundations of continents, and covered 
them with mountain ranges. This remark requires no limitation if 
we allow the requisite time, and connect with the power of growth 
such other agencies, soon to be explained, as have been at work in 
the Pacific since the reefs were there in progress. 
The death of the polyps about the base of a coral tree would expose 
it seemingly to immediate wear from the waters around it, and espe- 
cially as the texture is usually porous. But nature is not without an 
expedient to prevent a catastrophe that would be destructive to a large 
part of growing zoophytes, and would prevent the indefinite increase 
just explained. ‘The dead surface becomes the resting-place of num- 
berless small incrusting species of corals, besides Nullipores, Serpulas, 
and some molluscs. In many instances the lichen-like Nullipore 
grows at the same rate with the rate of death in the zoophyte, and 
keeps itself up to the very limit of the living part. The dead trunk 
of the forest becomes covered with lichens and fungi, or in tropical 
climes, with other foliage and various foreign flowers: so among the 
