ons
are undoubted, but they require a centre to attract them, and the
difficulty is to account for the beginning of a collecting centre or
planetary nucleus. Moreover, meteorites are sporadic and erratic,
scattered hither and thither rather than collecting into unit-bodies. As
Professor Chamberlin says, "meteorites have rather the characteristics
of the wreckage of some earlier organisation than of the parentage of
our planetary system." Several other theories have been propounded to
account for the origin of the earth, but the one that has found most
favour in the eyes of authorities is that of Chamberlin and Moulton.
According to this theory a great nebular mass condensed to form the sun,
from which under the attraction of passing stars planet after planet,
the earth included, was heaved off in the form of knotted spiral nebulae,
like many of those now observed in the heavens.

Of great importance were the "knots," for they served as collecting
centres drawing flying matter into their clutches. Whatever part of the
primitive bolt escaped and scattered was drawn out into independent
orbits round the sun, forming the "planetesimals" which behave like
minute planets. These planetesimals formed the food on which the knots
subsequently fed.

The Growth of the Earth

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, ab