ON CHEMICAL GEOLOGY. 477 
come down, and, by dissolving the previously deposited chloride of sodium, give rise to 
an ocean which, from the first, would be intensely salt. 
It has been ascertained from astronomical experiments that the mean density of the 
earth is about 54 ; now the average specific gravity of the substances forming the 
external crust is found to be about 275 ; and hence it follows that the internal portion 
of the earth must be composed of matters of very great density. This increase of den¬ 
sity towards the centre has usually been supposed to be due to pressure, but this sup¬ 
position seems to be inadequate to account altogether for this fact, which is probably, 
in part, to be attributed to an accumulation of heavy metals at the centre of the earth 
as well as to pressure. The crust of the surface, shortly after its solidification, became 
cracked and distorted, in consequence of contraction, so giving rise to mountain ranges, 
valleys, and other inequalities of outline. 
Since this Genetic Period all changes which have taken place have been effected by 
precisely the same forces as are now to be witnessed in operation ; wear and denudation 
took place as at the present day by the action of rains, rivers, and the ocean. In the 
primaeval atmosphere, however, the larger proportion of carbonic acid would assist 
materially in the disintegration of rocks, converting their alkalies into carbonates, which 
would be washed out together with the sulphides ; and these last again, by oxidatim, 
would furnish sulphates. The result of this disintegration would be to form the various 
argillaceous and calcareous deposits of the period which, in their turn, would undergo 
removal by wear. At about this stage organized beings would appear upon the scene, 
and considerably influence subsequent changes: vegetation, probably appearing first, 
would prepare the way for animals by fixing from the atmosphere a considerable pro¬ 
portion of carbon from the carbonic acid with which it was overcharged. 
Much discussion has taken place at different times with respect to the origin of 
granites, but it can be shown that the specific gravity of the quartz contained in granite 
does not necessarily point to an aqueous origin, since quartz crystals taken from vol¬ 
canic brachytes have been found to possess a specific gravity—2‘6—identical with that 
of ordinary rock crystals. 
From personal experience in the field and with the microscope, the lecturer has satis¬ 
fied himself that many granites and gneisses have been formed by the disintegration of 
older rocks ; the de hr is have been stratified by aqueous agency, and subsequently have 
undergone reconsolidation. Granites of igneous origin also undoubtedly exist, and the 
arguments which have been brought forward against such a view are answerable with¬ 
out difficulty. The objections that have been urged are principally that the specific 
gravity of the quartz is 2'6, and that the rock contains water. This, however, is not 
incompatible with the definition of igneous agency as laid down by the lecturer, the 
granite referred to having solidified at a heat far below the fusing-points of its consti¬ 
tuent minerals, and at such a pressure as to enable them to entangle and retain a small 
amount (£ to \ per cent.) of aqueous vapour, which naturally must have been present 
during its liquefaction. 
It has also been urged that in granite it sometimes happens that the less fusible 
minerals are not the first to crystallize ; here again it is represented in reply that in the 
lava from Vesuvius crystals of refractory leucite are frequently found sitting upon the 
easily fusible augite. 
That there still exists in the interior of the earth a portion which retains its fluidity, 
and that this is the source of volcanic rocks, we have evidence in the outpourings of 
volcanoes on different parts of the earth’s surface and in the occasional elevation and 
depression of parts of the crust. And again, the identity in chemical and physical cha¬ 
racters of all volcanic rocks proves to us that there are not, as some suppose, two or more 
chambers in the interior, but that volcanic eruptions must be produced from one and the 
same deep-seated source. 
Metamorphism, or that change of structure and texture which has been effected in 
many rocks of all ages, is represented by the lecturer as resulting from the operation of 
several distinct forces or combinations of forces, which may be classified in six divisions : 
namely, heat alone; pressure alone; heat in conjunction with chemical action ; aqueous 
action assisted by heat and pressure ; gasolytic action ; and lastly a combination of two 
or more of the above agencies. 
When a calcareous or ferruginous slate or clay is heated, a complete change is wrought 
both in the chemical constitution and in the appearance of the rock; and artificially 
