cataracts,--the silt and sand rest on
what is known as the "marine" or nummulitic limestone,--a formation of
great extent, for it runs into the Nubian desert on the one hand, and
into the Libyan desert on the other; and which, though it abounds in the
animalcules of the European chalk, is held to belong, in at least its
upper beds, which are charged with nummulites, to the earlier Eocene.
Over this marine limestone there rests a newer formation, of later
Tertiary age, which contains the casts of sea shells, and whole forests
of dicotyledonous trees, converted into a flint-like chert; and over all
repose the sands and gravels of the desert. Underneath the silt of the
river, then, and the sand of the desert, lie these two formations of the
Tertiary division. The lower, which is of great thickness, must have
been of slow formation. It is composed almost exclusively, in many
parts, of microscopic animals, and abounds in others in fossil
shells,--nautili, ostreadae, turritella, and nummulites, with corals,
sponges, the remains of crustacea, and the teeth of fishes. And between
the period of its deposition and that of the formation which rests upon
it the surface of what is now Egypt must have been elevated over the
surface of the sea, to be covered, in the course of ages, by great
forests, which, ere the land assumed its present form and level, were
submerged by another oscillation of the surface, and petrified amid beds
of a siliceous sand at the bottom of the ocean. Nor is the underlying
marine limestone by any means the oldest of the sedimentary rocks of
Egypt. It rests on a sandstone of Permian or Triassic age; the sandstone
rests, in turn, on the famous Breccia de Verde of Egypt; and the Breccia
on a group of Azoic rocks, gneisses, quartzes, mica schists, and clay
slates, that wrap round the granitic nucleus of Syene. The formations of
Egypt constitute a well-determined part of that great series of systems
which compose the upper portion of the earth's crust: its silt is by far
the most inconsiderable of its deposits; and if five thousand six
hundred and fifty years were exhausted in laying down layer after layer
of the twenty feet which form _its_ average thickness, what enormous
periods must we not demand in addition for the laying down of the forest
formation, of the marine limestone formation, of the New Red Sandstone
formation, of the Breccia de Verde formation, and, in short, for the
some ten miles of fossiliferous