
“Secr. i] THE EARTH IN THE SOLAR SYSTEM. —.7 
geological history is to be compiled from direct evidence furnished 
by the rocks of the earth, it cannot begin at the beginning of things, 
‘but must be content to date its first chapter from the earliest period 
_ of which any record has been preserved among the rocks. 
Nevertheless, though geology in its usual restricted sense has 
been, and must ever be, unable to reveal the earliest history of our 
planet, it no longer ignores, as mere speculation, what is attempted 
in this subject by its sister sciences. ‘Astronomy, physics, and che- 
mistry have in late years all contributed to cast much light on the 
earlier stages of the earth’s existence, previous to the beginning 
of what is commonly regarded as geological history. Whatever ex- 
tends our knowledge of the former conditions of our globe may be 
_ legitimately claimed as part of the domain of geology. If Geology 
therefore is to continue worthy of its name as the science of the 
earth, it must take cognisance of these recent contributions from 
other sciences. It can no longer he content to begin its annals with 
the records of the oldest rocks, but must endeavour to grope its way 
through the ages which preceded the formation of any rocks. 
Thanks to the results achieved with the telescope, the spectroscope, 
and the chemical laboratory, the story of these earliest ages of our 
earth is every year becoming more definite and intelligible. 
J. RELATIONS OF THE EARTH IN THE SOLAR SYSTEM. 
As a prelude to the study of the structure and history of the 
earth, some of the general relations of our planet to the solar 
system may here be noticed. The investigations of recent years 
showing the community of substance between the different members 
of that system, have revived and given a new form and meaning 
to the well-known nebular hypothesis of Kant, Laplace and 
W. Herschel, which sketched the progress of the system from the state 
of an original nebula to its existing condition of a central incandes- 
~ cent sun with surrounding cool planetary bodies. According to this 
hypothesis, the nebula, originally diffused at least as far as the 
furthest member of the system, began to condense towards the 
centre, and in so doing threw off or left behind successive rings 
which on disruption and further condensation assumed the form of 
planets, sometimes with a further formation of rings, which in the 
ease of Saturn remain, though in other planets they have broken up 
and united into satellites. 
Accepting this view, we should expect the matter composing the 
various members of the solar system to be everywhere nearly the 
same. The fact of condensation round centres, however, indicates 
at least differences of density throughout the nebula. That the 
materials composing the nebula may have arranged themselves ac- 
cording to their respective densities, the lightest occupying the 
exterior and the heaviest the interior of the mass, is suggested by a 
zomparison of the densities of the various planets. ‘These densities 
