-summit only are these fossils known to abound, but sometimes
in layer below layer of the mountain, from top to bottom, through
thousands of feet of rock.

[Illustration: FOSSIL SHELLS.]

This may well seem puzzling at first sight. Fossils of sea-creatures
on a mountain-top are startling enough; yet hardly so startling as the
thought of fossils _inside_ that mountain. How could they have found
their way thither?

The difficulty soon vanishes, if once we clearly understand that all
these thousands of feet of rock were built up slowly, layer after
layer, when portions of the land lay deep under the sea. Thus _each
separate layer_ of mud or sand or other material became in its turn
the _top layer_, and was for the time the floor of the ocean, until
further droppings of material out of the waters made a fresh layer,
covering up the one below.

While each layer was thus in succession the top layer of the building,
and at the same time the floor of the ocean, animals lived and died
in the ocean, and their remains sank to the bottom, resting upon the
sediment floor. Thousands of such dead remains disappeared, crumbling
into fine dust and mingling with the waters, but here and there one
was caught captive by the half-liquid mud, and was quickly covered and
preserved from decay. And still the building went on, and still layer
after layer was placed, till many fossils lay deep down beneath the
later-formed layers; and when at length, by slow or quick upheaval of
the ground, this sea-bottom became a mountain, the little fossils were
buried within the body of that mountain. So wondrously the matter
appears to have come about.

* * * * *

Another difficulty with respect to the stratified rocks has to be
thought of. All these layers or deposits of gravel, sand, or earth, on
the floor of the ocean, would naturally be horizontal--that is, would
lie flat, one upon another. In places the ocean-floor might slant, or
a crevice or valley or ridge might break the smoothness of the
deposit. But though the layers might partake of the slant, though the
valley might have to be filled, though the ridge might have to be
surmounted, still the general tendency of the waves would be to level
the dropping deposits into flat layers.

Then how is it that when we examine the strata of rocks in our
neighborhood, wherever that neighborhood may be, we do not find them
so arranged? Here, it is true, the lines for a spa