GEOLOGY OF THE INNER EARTH—GREGORY. 315 
driving all living beings toward the equator, until at length the whole 
earth, like the moon, would become lifeless through cold, as it had 
once been uninhabitable through heat. This theory has permanently 
impressed itself on geological terminology; and its corollaries, secular 
refrigeration and the contortion of the shrinking crust, once domi- 
nated discussions concerning climatic history and the formation of 
mountain chains. This nebular hypothesis, however, we are now told, 
is mathematically improbable, or even impossible; and it is only con- 
sistent with the facts of geology on the assumption that, in propor- 
tion to the age of the world, the whole of geological time is so insig- 
nificant that the secular refrigeration during it is quite inappreciable; 
hence geology can no more confirm or correct the theory that a stock- 
breeder could refute evolution by failing to breed kangaroos into cows 
in a single lifetime. 
The theory of the gaseous nebula has been probably of more 
hindrance than help to geologists; its successors, the meteoritic 
hypothesis of Lockyer and the planetismal theory of Chamberlin, are 
of far more practical use to us, and they give a history of the world 
consistent with the actual records of geology. According to Sir 
Norman Lockyer’s meteoritic hypothesis, nebule comets and many 
so-called stars consist of swarms of meteorites which, though nor- 
mally cold and dark, are heated by repeated collisions, and so become 
luminous. They may even be volatilized into glowing meteoric 
vapor; but in time this heat is dissipated, and the force of gravity 
condenses a meteoritic swarm into a single globe. Some of the 
swarms are, says Lockyer, “truly members of the solar system,” and 
some of them travel around the sun in nearly circular orbits, like 
planets. They may be regarded as infinitesimal planets, and so 
Chamberlin calls them planetismals. 
The planetismal theory is a development of the meteoritic theory, 
and presents it in an especially attractive guise. It regards meteorites 
as very sparsely distributed through space, and gravity as powerless 
to collect them into dense groups. So it assigns the parentage of the 
solar system to a spiral nebula composed of planetismals, and the 
planets as formed from knots in the nebula, where many planetismals 
had been concentrated near the intersections of their orbits. These 
groups of meteorites, already as solid as a swarm of bees, were then 
packed closer by the influence of gravity, and the contracting mass 
was heated by the pressure, even above the normal melting point of 
the material, which was kept rigid by the weight of the overlying 
layers. 
This theory has the recommendation of being consistent with the 
history of the earth as interpreted by geology. For whereas the 
nebular hypothesis represents the earth as having been originally 
intensely hot, and having persistently cooled, yet geological records 
