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Young Folks' Library: Wonders of Earth, Sea and Sky

Some Records Of The Rocks
(From A First Book in Geology.)
The geologist cannot find his way back in the record of the great stone book, to the far-
off day when life began. The various changes that come over rocks from the action of
heat, of water, and of pressure, have slowly modified these ancient beds, so that they no
longer preserve the frames of the animals that were buried in them.
These old rocks, which are so changed that we cannot any longer make sure that any
animals lived in them, are called the "archæan," which is Greek for ancient. They were
probably mud and sand and limestone when first made, but they have been changed to
mica schists, gneiss, granite, marble, and other crystalline rocks. When any rock becomes
crystalline, the fossils dissolve and disappear, as coins lose their stamp [pg 78] and form when
they are melted in the jeweller's gold-pot.
These ancient rocks that lie deepest in the earth are very thick, and must have taken a
great time in building; great continents must have been worn down by rain and waves in
order to supply the waste out of which they were made. It is tolerably certain that they
took as much time during their making as has been required for all the other times since
they were formed. During the vast ages of this archæan the life of our earth began to be.
We first find many certain evidences of life in the rocks which lie on top of the archæan
rock, and are known as the Cambriani and Silurian periods. There we have creatures akin
to our corals and crabs and worms, and others that are the distant kindred of the cuttle-
fishes and of our lamp-shells. There were no backboned animals, that is to say, no land
mammals, reptiles, or fishes at this stage of the earth's history. It is not likely that there
was any land life except of plants and those forms like the lowest ferns, and probably
mosses. Nor is it likely that there were any large continents as at the present time, but
rather a host of islands lying where the great lands now are, the budding tops of the
continents just appearing above the sea.
Although the life of this time was far simpler than at the present day, it had about as great
variety as we would find on our present sea-floors. There were as many different species
living at the same time on a given surface.
The Cambrian and Silurian time—the time before the coming of the fishes—must have
endured for [pg 79] many million years without any great change in the world. Hosts of species
lived and died; half a dozen times or more the life of the earth was greatly changed. New
species came much like those that had gone before, and only a little gain here and there
was perceptible at any time. Still, at the end of the Silurian, the life of the world had
climbed some steps higher in structure and in intelligence.
The next set of periods is known as the Devonian. It is marked by the rapid extension of
the fishes; for, although the fishes began in the uppermost Silurian, they first became
abundant in this time. These, the first strong-jawed tyrants of the sea, came all at once,
like a rush of the old Norman pirates into the peaceful seas of Great Britain. They made a
lively time among the sluggish beings of that olden sea. Creatures that were able to meet
feebler enemies were swept away or compelled to undergo great changes, and all the life
of the oceans seems to have a spur given to it by these quicker-formed and quicker-willed