476 
ON CHEMICAL GEOLOGY. 
Report of a Lecture by David Forbes , Esq., F.R.S., delivered before the Chemical 
Society , February 20th , 18(18. 
The rival Plutonic and Neptunian theories have long been employed by their respec¬ 
tive adherents in accounting for the characters and constitution of the oldest rocks, but 
they both partake of the serious defect of referring to one agency alone results which 
must have been brought about by the simultaneous operation of several distinct forces. 
In scarcely any instance does a theory which supposes only one agency account satisfac¬ 
torily for the phenomena of nature, several forces being invariably found to co-operate in 
their production. Moreover, the grand principle of the transmutation of force ought, in 
all such inquiries, to receive fair consideration. We know, for instance, that mechanical 
power is capable, when suitably applied, of producing heat, light, electricity, and even, 
indirectly, chemical action. Instances of this plurality of origin are plentiful enough 
in geological phenomena. For example, quartz may be of igneous, aqueous, or gaso- 
lytic origin ; it may occur iu nature as an igneous product in recent volcanic lavas, as 
an aqueous product in different forms deposited from solution, and from a gaseous con¬ 
dition by decomposition of the fluoride of silicon. Sulphur, copper, and other sub¬ 
stances furnish examples of similar modifications, and it is interesting to observe that 
these minerals are capable of artificial imitation. 
It will be necessary, in order to avoid confusion and misunderstanding, to define 
clearly at the commencement what is meant by igneous, aqueous, and gasolytic action, 
since these are terms which will be frequently made use of in the following observa¬ 
tions. By igneous action must be understood volcanic action perfectly similar to that 
which is going on in the volcanoes of the present day. It does not signify a mere dry 
fusion, but the action of heat accompanied and modified by that of vapours and gases. 
Large quantities of steam and other gaseous products are constantly observed to be 
ejected during the volcanic eruptions of our own times ; and there can be no doubt that 
these, and particularly aqueous vapours, play a very active and important part. Simi¬ 
larly, aqueous action is definable as the effects of water, including those simultaneously 
produced by the dissolved saline matter and gases assisted by heat. Gasolytic action is 
in like manner modified by heat and aqueous agency. 
Commencing as far back as astronomical and physical phenomena carry us, at that 
period, in fact, of the world’s history known as the “ Cosmogenetic Era,” we have 
reason to believe that the earth was at this period so heated, from some cause unknown 
to us, as to be in a state of fusion or liquefaction ; and around this molten ball must 
have existed a dense atmosphere, formed of the vapours of all the volatile matters of 
the globe. 
Inasmuch as, even at the present day, comparatively little is known of the physical 
circumstances affecting chemical reactions, the chemist may fairly be permitted to claim 
great latitude in the attempt at explaining the phenomena of this epoch. It is, how¬ 
ever, conceivable that iu the beginning the newly-created matters resulting from the 
exercise of the affinities of the elements present being, as has been supposed, in a state 
of fusion, would arrange themselves in successive zones or strata in the order of gravity, 
the most dense occupying the centre; this stratification doubtless being, to a certain 
extent, subject to interference from the influence of diffusion. The external portion 
would consist chiefly of acid silicates, floating upon a substratum more basic in charac¬ 
ter ; and this increase in the proportion of the metallic element, together with com¬ 
pounds of sulphur and arsenic, would continue with the increase of depth. Simulta¬ 
neously with these arrangements similar operations may be imagined to have taken 
place in the atmosphere, the lowest stratum of which would contain large quantities of 
the vapour of chloride of sodium. Of course, it may be regarded as probable that after 
a time, in consequence of diffusion having taken place, most of these zones of vapour 
and liquid would undergo intermixture to a considerable extent. Subsequently, radia¬ 
tion of heat being all this time in progress, the mass would be so far cooled as to enable 
a solid crust to form upon the surface ; the chloride of sodium (of which it is calculated 
there is sufficient to cover the whole surface of the globe in a layer ten feet thick) 
would, after a time, condense from the atmosphere into the solid state, forming a 
superficial deposit. Later, when the temperature was further reduced, the water would 
