MOLLUSCA. 51 
depth of four inches; and it is observed that the 
nodules of quartz and feldspar, which sometimes 
occur in the hard limestone of the pillars, are un- 
touched. 
Many theories have been invented to account for 
the singular power exercised by these animals, such 
as the following; that the animals entered the rock 
while it was in a soft and plastic state, and that it 
afterwards hardened around them—that the animal 
poured out some peculiar fluid which had the 
chemical property of dissolving the rock—that the 
latter was ground away by the roughnesses on 
the shells as they revolved, as if by the action of 
a rasp or file—that the minute particles of the stone 
were one by one separated and driven off by the 
force of currents of water, produced by vibrating 
cilia: but all these theories appear to be set aside 
by the discovery of Mr. Albany Hancock, one of 
the highest living authorities on the subject. This 
gentleman finds that the excavating instrument is 
the anterior portion of the animal, either the foot 
and the edges of the mantle, or the edges of the 
mantle solely. These organs are fitted for the 
office they are to perform, not only by their position 
and figure, and their phiability and muscular struc- 
ture,—made more than commonly muscular for the 
duty,—but also by being armed with a rough layer 
of numerous crystalline particles of various sizes 
and shapes, chiefly five- and six-sided, and all 
having one or more elevated points near the centre. 
These crystals are imbedded in the surface of the 
boring foot and thickened edges of the mantle ; 
and, consisting, probably, of silex or flint, either 
pure or in combination with some animal matter, 
they form a sort of file—superior, however, to any 
