Actions of Volcanoes. 289 
always an excuse for an incorrect chart, or, as in the case of the 
Alceste, for a bad reckoning, to assert that a new rock was 
found where the old one had been overlooked. 
On examining the soundings of the seas in which they lie, 
and on comparing their positions, it appears probable that the 
various dispositions, as well as the places of the reefs, are in a 
great measure determined by the forms of the submarine land, 
and that they are placed on the tops of the hills, or on the most 
elevated parts of the bottom. When they form straight or 
curved lines, the side of the submarine structure to windward, 
or that which is exposed to the breach of the sea, rises almost 
vertically in the manner of a wall; while to the leeward, they 
shelve gradually away, so as to deepen the water as they pro- 
ceed in this direction, when, at the other side they have reached 
its surface. It is supposed that there is here some design for 
effecting a purpose which it is thought that accident can 
scarcely have determined ; and that the intention of the animal 
in thus building up to the windward, was to procure shelter for 
continuing its productions to leeward. Whatever may be 
thought of that supposition, it is this abrupt manner of rising 
from the bottom which renders them so dangerous to ships; as, 
from deep soundings, they may in a moment be on shore, and 
almost without warning. 
When the groups are circular, there are some peculiarities in 
them, as well as in the results, which are worthy of notice. A 
number of detached rocks and islands are first observed, form- 
ing a chain, which becomes gradually united in different places, 
so as to hold out the prospect of its becoming continuous at 
some future day. All round this, on the outside, the water is 
deep, and the walls vertical; but within, it is found to shoal in 
different places, so as to convey the idea of a large platform, 
surrounded by an elevated margin, with a depression in the 
middle. In the smaller circles, when this process is completed, 
the reefs represent a circular basin. This basin continues to be 
salt, and is a receptacle for sea water for some time, during 
which it continues to grow shallower gradually, as the animals 
within it prolong their operations upwards. But as the water 
Vox. XIV. U 
