ngland. When we pass to the upper formation,
we find the holoptychius the most characteristic fossil.

These fossils are found in a degree of entireness which depends less on
their age than on the nature of the rock in which they occur. Limestone
is the preserving salt of the geological world, and the conservative
qualities of the shales and stratified clays of the Lower Old Red
Sandstone are not much inferior to limestone itself; while in the Upper
Old Red the beds of consolidated sand are much less conservative of
organic remains. The older fossils, therefore, can be described almost
as minutely as the existence of the present creation, whereas the newer
fossils exist, except in a few rare cases, as fragments, and demand the
powers of a Cuvier or an Agassiz to restore them to their original
combinations. On the other hand, while the organisms of the Lower Old
Red are numerous and well preserved, those of the Upper Old Red are much
greater in individual size. In short, the fish of the lower ocean must
have ranged in size between a stickleback and a cod; whereas some of the
fish of the ocean of the Upper Sandstone were covered with scales as
large as oyster shells, and were armed with teeth that rivalled in size
those of the crocodile.

_IV.--Fish as Nature's Last Word_

I will now attempt to present to the reader the Old Red Sandstone as it
existed in time--during the succeeding periods of its formation, and
when its existences lived and moved as the denizens of primeval oceans.
We pass from the cemetery with its heaps of bones to the ancient city
full of life and animation in all its streets and dwellings.

Before we commence our picture, two great geological periods have come
to their close, and the floor of the widely spread ocean is occupied to
the depth of many thousand feet by the remains of bygone existences. The
rocks of these two earlier periods are those of the Cambrian and
Silurian groups. The lower--Cambrian, representative of the first
glimmering twilight of being--must be regarded as a period of
uncertainty. It remains for future discoverers to determine regarding
the shapes of life that burrowed in its ooze or careered through the
incumbent waters.

There is less doubt respecting the existences of the Silurian rocks.
Four distinct platforms of being range in it, the one over the other,
like the stories of a building. Life abounded on all these platforms,
and in shapes the most wonderful. In the pe