mountains.

Coast ice, likewise, may transport rocks and earth. Springs also must be
considered as geological agents affecting the face of the globe.

But running water not only denudes it, but also creates land, for
lakes, seas, rivers are seen to form deltas. That Egypt was the gift of
the Nile was the opinion of the Egyptian priests, and there can be no
doubt that the fertility of the alluvial plain above Cairo, and the very
existence of the delta below that city, are due to the action of that
great river, and to its power of transporting mud from the interior of
Africa and depositing it on its inundated plains as well as on that
space which has been reclaimed from the Mediterranean and converted into
land. The delta of the Ganges and Brahmapootra is more than double that
of the Nile. Even larger is the delta of the Mississippi, which has been
calculated to be 12,300 square miles in area.

TIDES AND CURRENTS. The transporting and destroying and constructive
power of tides and currents is, in many respects, analogous to that of
rivers, but extends to wider areas, and is, therefore, of more
geological importance. The chief influence of the ocean is exerted at
moderate depths below the surface on all areas which are slowly rising,
or attempting, as it were, to rise above the sea; but its influence is
also seen round the coast of every continent and island.

* * * * *

We shall now consider the igneous agents that act on the earth's
surface. These agents are chiefly volcanoes and earthquakes, and we find
that both usually occur in particular parts of the world. At various
times and at various places within historical times volcanic eruptions
and earthquakes have both proved their potency to alter the face of the
earth.

The principal geological facts and theories with regard to volcanoes and
earthquakes are as follows.

The primary causes of the volcano and the earthquake are to a great
extent the same, and connected with the development of heat and chemical
action at various depths in the interior of the globe.

Volcanic heat has been supposed to be the result of the original high
temperature of the molten planet, and the planet has been supposed to
lose heat by radiation. Recent inquiries, however, suggest that the
apparent loss of heat may arise from the excessive local development of
volcanic action.

Whatever the original shape of our planet, it must in time have become
sph