, that while
these divisions may in certain cases be due to the above-mentioned
and, indeed, to a great variety of causes, they are in general best to
be explained by the action of earthquakes. Water being an exceedingly
elastic substance, an earthquake passes through it with much greater
speed than it traverses the rocks which support the ocean floor. The
result is that, when the fluid and solid oscillate in the repeated
swingings which a shock causes, they do not move together, but rub
over each other, the independent movements having the swing of from a
few inches to a foot or two in shocks of considerable energy.

When the sea bottom and the overlying water, vibrating under the
impulse of an earthquake shock, move past each other, the inevitable
result is the formation of muddy water; the very fine silt of the
bottom is shaken up into the fluid, which afterward descends as a
sheet to its original position. It is a well-known fact that such
muddying of water, in which species accustomed to other conditions
dwell, inevitably leads to their death by covering their breathing
organs and otherwise disturbing the delicately balanced conditions
which enable them to exist. We find, in fact, that most of the tenants
of the water, particularly the forms which dwell upon the bottom, are
provided with an array of contrivances which enable them to clear away
from their bodies such small quantities of silt as may inconvenience
them. Thus, in the case of our common clam, the breathing organs are
covered with vibratory cilia, which, acting like brooms, sweep off any
foreign matter which may come upon their surfaces. Moreover, the
creature has a long, double, spoutlike organ, which it can elevate
some distance above the bottom, through which it draws and discharges
the water from which it obtains food and air. Other forms, such as the
crinoids, or sea lilies, elevate the breathing parts on top of tall
stems of marvellous construction, which brings those vital organs at
the level, it may be, of three or four feet above the zone of mud. In
consequence of the peculiar method of growth, the crinoids often
escape the damage done by the disturbance of the bottom, and thus form
limestone beds of remarkable thickness; sometimes, indeed, we find
these layers composed mainly of crinoidal remains, which exhibit only
slight traces of partings such as we have described, being essentially
united for the depth of ten or twenty feet. Where the layer