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pounds of silicon with fluorine, chlorine, &c. ; and it even might 
be regarded as an organic product, when produced from the de- 
composition of the silicic ethers. 
His experience in the field would further provide him with 
abundant cases of crystallised silica or quartz, as an igneous 
product occurring in the lavas from volcanoes ; as an aqueous 
product crystallised from solution, or proceeding from the de- 
composition of mineral silicates ; as a gasolitic product in the 
form of tubes, evidently resulting from the decomposition of its 
fluorine compounds ; whilst the carapaces and other parts of 
infusoria, &c., present silica in a form w^hich owes its appear- 
ance to the action of organic life. 
Numerous other examples might be cited to show that this 
is, if anything, the rule and not the exception ; and the con- 
clusion to be deduced from their study is self-evident, namely, 
that it is impossible to be over-cautious in attributing the form- 
ation of minerals, or of the rock masses in 'which they occur, to 
any one cause, to the exclusion of other agencies. 
This must be constantly borne in mind when enquiring into 
the formation of minerals or rocks, and no generalisations based 
on single experiments should be hastily assumed as correct; 
for example, it is well known that a precipitate or deposit of 
carbonate of lime is formed when a solution of carbonate of 
soda is added to one of chloride of calcium, yet the chemist is 
not entitled to rush to the conclusion, either that all limestones 
had been so formed, or even that the lime which they are known 
to contain had ever been in the state of chloride of calcium, at 
least not until further investigation, both in the laboratory and 
field, did satisfy him that it could not have been otherwise ; on 
the contrary, however, such an enquiry proves satisfactorily that 
even if, in some rare instances, limestones may have been formed 
by the direct precipitation of chloride of calcium by the carbonate 
of soda, that this is the exception and not the rule, which shows 
them to have been secreted by the action of organic life, from 
the lime salts held in solution in the ocean, and further points 
out how other limestones have originated in the deposition of 
carbonate of lime previously held in solution in water charged 
with carbonic acid. 
Just as well might it be inferred that the beds of gypsum had 
been produced by a stream of sulphuric acid flowing into an 
ocean of lime salts, because sulphuric acid does precipitate lime 
as a sulphate ; but it is well known that some gypsum deposits 
are of quite different origin to others, in some cases being 
merely the result of the evaporation of sea- water, which always 
contains a large amount of sulphate of lime; in others of the 
conversion, in situ, of limestones by gasolytic action or infil- 
tration — whilst occasionally they may have been the result of 
