It has been calculated that the newborn earth--the "earth-knot" of
Chamberlin's theory--had a diameter of about 5,500 miles. But it grew
by drawing planetesimals into itself until it had a diameter of over
8,100 miles at the end of its growing period. Since then it has shrunk,
by periodic shrinkages which have meant the buckling up of successive
series of mountains, and it has now a diameter of 7,918 miles. But
during the shrinking the earth became more varied.
A sort of slow boiling of the internally hot earth often forced molten
matter through the cold outer crust, and there came about a gradual
assortment of lighter materials nearer the surface and heavier materials
deeper down. The continents are built of the lighter materials, such as
granites, while the beds of the great oceans are made of the heavier
materials such as basalts. In limited areas land has often become sea,
and sea has often given place to land, but the probability is that the
distinction of the areas corresponding to the great continents and
oceans goes back to a very early stage.
The lithosphere is the more or less stable crust of the earth, which may
have been, to begin with, about fifty miles in thickness. It seems that
the young earth had no atmosphere, and that ages passed before water
began to accumulate on its surface--before, in other words, there was
any hydrosphere. The water came from the earth itself, to begin with,
and it was long before there was any rain dissolving out saline matter
from the exposed rocks and making the sea salt. The weathering of the
high grounds of the ancient crust by air and water furnished the
material which formed the sandstones and mudstones and other sedimentary
rocks, which are said to amount to a thickness of over fifty miles in
all.
§ 3