THE FIRST CHAPTER OF THE GEOLOGICAL RECORD. 
353 
found that neither the molten sphere nor the atmosphere sur- 
rounding it was of uniform character throughout ; but owing 
to both of them being made up of a number of dissimilar sub- 
stances, the first impulse of the newly-formed compounds 
would be to obey the laws of gravity by arranging themselves 
more or less completely in strata, or more correctly speaking, 
zones, in accordance with their respective densities, and the 
study of the composition of the rocks now forming the earth’s 
exterior, and of those brought up from its depths by volcanic 
forces, along with that of the relative specific gravities of the 
parts accessible to our observation as compared to the density 
of the earth’s mass as a whole, leads to the deduction that the 
molten sphere might at this period of its history be regarded 
as composed of some three great zones (probably with sub- 
zones), having the following general mineralogical characters: — 
1st. An exterior of molten rock of comparatively little 
density which consisted of silicates, in which an excess of silica 
was to a great extent combined with alumina and alcali, but 
containing very little of the other bases, such as lime, magnesia, 
oxide of iron, &c. 
2nd. A middle zone, also of molten rock (silicates), con- 
siderably heavier than the former, and in which the silica, 
present in minimum proportion, existed in combination with a 
large amount of the bases, lime, magnesia, oxide of iron, and 
alumina, with but comparatively little potash, and, 
3rd. A central nucleus of very much greater density and of 
metallic nature, the outer part consisting of compounds cf the 
heavy metals with sulphur, arsenic, &c., whilst in the centre the 
metals themselves are probably in a free state, or as alloys. 
The constitution of the sphere of molten matter as thus 
arranged would now present a general character of stability 
maintained even after its solidification, due to the loss of heat 
radiated from its surface, and the cooling action of the external 
air, had commenced. In the atmosphere, however, the arrange- 
ment of the gases and vapour in zones would be much less per- 
manent, as by degrees the zones would be more or less broken 
up by the tendency which gases have to diffuse themselves 
throughout one another, as well as the condensation in succes- 
sion of the different vapours contained in it, in proportion as 
the temperature of the whole became more and more lowered. 
In the first instance, however, that stratum of the atmosphere 
next to the earth would be composed of dense vapours of such 
compounds as are only volatile at very high temperatures, 
amongst which several of the chlorides, and especially the 
chloride of sodium or common salt would be most prominent ; 
above this a great zone of carbonic acid gas would prevail, then 
one of nitrogen with possibly the admixture of some oxygen 
YOL. XI. — NO. XLV. A A 
