THE INFANCY OF THE EARTH
The long Archaean period, into which half the story of the earth
is so unsatisfactorily packed, came to a close with a
considerable uplift of the land. We have seen that the earth at
times reaches critical stages owing to the transfer of millions
of tons of matter from the land to the depths of the ocean, and
the need to readjust the pressure on the crust. Apparently this
stage is reached at the end of the Archaean, and a great rise of
the land --probably protracted during hundreds of thousands of
years--takes place. The shore-bottoms round the primitive
continent are raised above the water, their rocks crumpling like
plates of lead under the overpowering pressure. The sea retires
with its inhabitants, mingling their various provinces,
transforming their settled homes. A larger continent spans the
northern ocean of the earth.
In the shore-waters of this early continent are myriads of living
things, representing all the great families of the animal world
below the level of the fish and the insect. The mud and sand in
which their frames are entombed, as they die, will one day be the
"Cambrian" rocks of the geologist, and reveal to him their forms
and suggest their habits. No great volcanic age will reduce them
to streaks of shapeless carbon. The earth now buries its dead,
and from their petrified remains we conjure up a picture of the
swarming life of the Cambrian ocean.
A strange, sluggish population burrows in the mud, crawls over
the sand, adheres to the rocks, and swims among the thickets of
sea-weed. The strangest and most formidable, though still too
puny a thing to survive in a more strenuous age, is the familiar
Trilobite of the geological museum; a flattish animal with broad,
round head, like a shovel, its back covered with a three-lobed
shell, and a number of fine legs or swimmers below. It burrows in
the loose bottom, or lies in it with its large compound eyes
peeping out in search of prey. It is the chief representative of
the hard-cased group (Crustacea) which will later replace it with
the lobster, the shrimp, the crab, and the water-flea. Its
remains form from a third to a fourth of all the buried Cambrian
skeletons. With it, swimming in the water, are smaller members of
the same family, which come nearer to our familiar small
Crustacea.
Shell-fish are the next most conspicuous inhabitants. Molluscs
are already well represented, but the more numerous are the more
elementary Brachiopods ("lampshells"), which come next to the
Trilobites in number and variety. Worms (or Annelids) wind in and
out of the mud, leaving their tracks and tubes for later ages.
Strange ball or cup-shaped little animals, with a hard frame,
mounted on stony stalks and waving irregular arms to draw in the
food-bearing water, are the earliest representatives of the
Echinoderms. Some of these Cystids will presently blossom into
the wonderful sea-lily population of the next age, some are
already quitting their stalks, to become the free-moving
star-fish, of which a primitive specimen has been found in the
later Cambrian. Large jelly-fishes (of which casts are preserved)
swim in the water; coral-animals lay their rocky foundations, but
do not as yet form reefs; coarse sponges rise from the floor; and
myriads of tiny Radiolaria and Thalamophores, with shells of
flint and lime, float at the surface or at various depths.
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