had
previously been assumed from the facts known; and that all attempts to
describe, from positive knowledge, anything like a progression of
being on the face of our globe, were at least premature. Professor
Owen had, at first, scarcely any hesitation in pronouncing the
footprints to be those of tortoises; but he afterwards changed his
views, and expressed his belief that the impressions had been produced
by small crustacean animals. Thus the views previously entertained
regarding the invertebrate character of the _fauna_ of the Silurian
epoch, have ultimately remained unaffected, so far as these Potsdam
slabs are concerned.

Slabs of sandstone and shale often retain what is called the
ripple-mark--that is, the corrugation of surface produced by the
gentle agitation of shallow water over sand or mud. We can see these
appearances beneath our feet, as we walk over the pavement of almost
any of our cities. Such slabs are also occasionally marked by
irregular protuberances, being the casts of hollows or cracks produced
in ancient tide-beaches by shrinkage. In many instances, the
footprints of animals are marked by such lines passing through them,
shewing how the beach had dried and cracked in the sun after the
animals had walked over it. In the quarries at Stourton, in Cheshire,
some years ago, a gentleman named Cunningham observed slab surfaces
mottled in a curious manner with little circular and oval hollows, and
these were finally determined to be the impressions produced by
rain--the rain of the ancient time, long prior to the existence of
human beings, when the strata were formed! Since then, many similar
markings have been observed on slabs raised from other quarries, both
in Europe and America; and fossil rain-drops are now among the settled
facts of geology. Very fine examples have been obtained from quarries
of the New Red Sandstone at Newark and Pompton, in New Jersey. Sir
Charles Lyell has examined these with care, and compared them with the
effects of modern rain on soft surfaces of similar materials. He says,
they present 'every gradation from transient rain, where a moderate
number of drops are well preserved, to a pelting shower, which, by its
continuance, has almost obliterated the circular form of the cavities.
In the more perfectly preserved examples, smaller drops are often seen
to have fallen into cavities previously made by larger ones, and to
have modified their shape. In some cases of partial interfe