31 / 
gentleman’s considering it to be impossible, that the igneous origin 
of fossils could be recorded in plainer language, than by the phe- 
nomenon displayed in the formation of the agate. Concluding, as 
every appearance, indeed, warrants, that the progress of consoli- 
dation, both in the solid and hollow agates, has been from the 
circumference inwards, he says, “ Now it must be considered that 
these coats are highly consolidated ; that they are of very pure sili- 
cious matter, and are utterly impervious to every substance which 
we know of, except light and heat. It is plain, therefore, that 
whatever, at any time, during the progress of consolidation, was 
contained within the coats already formed, must have remained 
there as long as the agate was entire, without the least probability 
of escape. But nothing is found within the coats of the agate, 
save its own substance j therefore no extraneous substance, that is 
to say no solvent, was ever included within them. The fluidity of 
the agate was therefore simple, and unassisted by any menstruum. 
In this argument, nothing appears to me wanting, that is necessary 
to the perfection of a physical, I had almost said of a mathematical 
demonstration*.” 
To determine the degree of force with which this argument op- 
poses the opinion, that these, and similar bodies, have originated 
from a solution of silex, in an aqueous menstruum, will, I trust, be 
best done, by endeavouring to point out those which appear to be 
the most probable modes, in which these bodies may have been 
formed. 
Silicious and agatine nodules are formed by a solid silicious mass, 
or the silicious matter is confined to a crust, varying in its thickness. 
This latter kind is termed a geode, and is either hollow and empty, 
or contains substances different from the crust itself, such as sand, 
indurated clay, and even water. If a loose hard stone is contained 
* Illustrations of the Huttonian Theory, p. 79. 
