THE STUDY OF CHEMICAL GEOLOCY. 
251 
actions of heat, of water, and of gases are found to be combined, 
each playing a more or less prominent part. Yet there can 
be no misunderstanding or confusing the precise meaning to 
be attached to each term. 
In igneous or volcanic action, whilst the effects of heat pre- 
dominate, the presence of heated steam and gases exercises a 
most important influence in modifying the results : and in this 
case the water present is in the form of steam. 
In aqueous action, on the other hand, the water acts as a 
liquid, not as a vapour, and is the main agency. Yet the effects 
of the gaseous and solid constituents, as well as of its tempera- 
ture, must be taken into full consideration. 
The immense volumes of steam emitted by volcanoes during 
their outbursts, would naturally prepare the observer to expect 
that some portion might become entangled in the lava, and thus 
account for the microscopic cavities containing water frequently 
found in volcanic products ; whilst at the same time he would 
not consider the presence of microscopic water cavities in the 
older rocks as proving any dissimilarity of origin, or as neces- 
sarily demonstrating them to be of aqueous formation, as has 
been advanced by some writers on the subject. 
In like manner, when, as Bunsen and Laurent have shown 
experimentally in the laboratory, that hydrous silicates and 
borates may be formed and retain their water, even when ex- 
posed to temperatures above the melting point of silver, the 
student must be convinced that water can enter into and be 
retained in combination even at a very high heat, and probably 
to his surprise, finds himself driven to the conclusion, that after 
all, fire and water are not so antagonistic or incompatible in 
nature, as they at first sight would appear to be. 
It is but seldom or ever that any change in the crust of our 
globe can be traced to any one sole cause, and it seems quite 
impossible for the impartial or unbiassed observer to come to any 
other conclusion than that most, if not all, the phenomena of 
nature, are due to a combination of forces, and that the same 
identical phenomenon may at times be the result of agencies 
totally different from those which at other times have given rise 
to its appearance. 
The chemical geologist, when enquiring into the mode of 
formation of the mineral constituents of rock masses, must, both 
in the field and laboratory, soon convince himself of the truth 
of this assertion. Thus, taking, for example, the most widely 
spread of all substances, silica, he finds that he can produce it 
in his laboratory by many totally distinct processes : as an 
igneous product by the oxidation of silicon at high tempera- 
tures ; as an aqueous product by the decomposition of silicates ; 
as a gasolytic product from the decomposition of the gaseous com- 
