PRESIDENTIAL ADDRESS. 4.93 
improbable, or even impossible; and it is only consistent with the facts of Geology 
on the assumption that, in proportion to the age of the world, the whole of 
geological time is so insignificant that the secular refrigeration during it is quite 
inappreciable ; hence Geology can no more confirm or correct the theory than a 
stockbreeder 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 uf Geology. According 
to Sir Norman Lockyer’s meteoritic hypothesis, nebulee comets and many so-called 
stars consist of swarms of meteorites which, though normally cold and dark, are 
heated by repeated collisions, and so become luminous. They may even be 
volatilised into glowing meteoritic vapour; 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 distri- 
buted 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 repre- 
sents the earth as having been originally intensely hot, and having persistently 
cooled, yet geological records show that an extensive low-level glaciation occurred 
in Cambrian times in low latitudes in South Australia ;! indeed, it seems probable 
that, in spite of many great local variations, the average climate of the whole 
world has remained fairly constant throughout geological time. Whereas it has 
often been represented, in accordance with the nebular theory, that volcanic action 
has steadily waned, owing to the lowering of the earth’s internal fires and the 
constant thickening of its crust, yet epochs of intense volcanic action have recurred 
throughout the world’s history, separated by periods of comparative quiescence. 
Whereas it has been assumed, as a corollary to the nebular theory, that the force 
which uplifted mountain chains was the crumpling of the crust owing to the 
contraction of the internal mass, yet observation reveals that the crust has been 
corrugated, and fold mountains formed by contraction to an extent far greater 
than secular cooling can explain, 
2. The Materials of the Inner Earth.—This planetismal hypothesis is not only 
consistent with geological records, but also with the known facts as to the internal 
composition of the earth and the structure of extra-terrestrial bodies as revealed 
by meteorites. Meteorites are of two main kinds—the meteoric irons, which 
consist of nickel iron, and stony meteorites, which are composed of basic minerals. 
Some of the stony meteorites have been shattered into fault breccias, showing that 
they are fragments of larger bodies which were subject to internal movements, 
like those that have formed crush-conglomerates in the crust of the earth. Those 
stony meteorites, therefore, both in composition and structure resemble the rocks 
in the comparatively shallow fracture zone of the earth’s crust. The nickel-iron 
meteorites, on the other hand, represent the barysphere beneath the crust. 
The earth appears to consist of material similar to that of the two types of 
meteorites ; but whether the proportions of the two materials in the earth represent 
‘ As shown by the work of Professor Howchin, of Adelaide. 
