Man's physical universe

xanabras

UNIT III

SECTION 6

THE HISTORY OF THE EARTH IS DIVIDED INTO

ERAS, PERIODS, AND EPOCHS

Introduction

The hills are shadows, and they flow

From form to form, and nothing stands;

They melt like mists, the solid lands;

Like clouds they shape themselves and go.

— Source unknown.

If we should return to our favorite resort near some mountain stream

or lake or seashore in the year 3000, very little change would be noted.

We might find that the stream had cut a little

deeper channel, that

the shores of the lake or ocean had moved a few hundred feet or so, but

the general contour of the mountains and the configurations of the

coast would be familiar.

But if we should return several million years later, nothing would

seem to be the same. Where we had known mountains, plains might

exist. New mountain ranges might be observed, and the oceans might

now be covering large sections which were formerly high dry land. Our

dams and skyscrapers might long since have been ground to powder

under a glacier a couple of miles thick.

Suppose that we were to excavate a portion of a city dump that

had been gradually built up over a period of two hundred years. In

the lower layers we might find old knives, spinning-wheel parts, horseshoes,

and handmade square nails. In other layers, sewing-machine

parts, cylindrical phonograph records, tin cans, machine-made wire

nails, aluminum utensils, and radio tubes would appear for the first

time. A study of the contents of these different strata in the city dump

would enable us to piece together a fairly accurate story of the developments

which took place during the two-hundred-year period. In this

same way archeologists are studying civilizations which existed thou-

many of

sands of years ago by excavating the ruins of ancient cities,

which were found to rest in turn on six or more layers of previous ruins.

The geologist obtains a story of the history of the earth by a similar

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