317 



tleman's considering it to be impossible, that the igneous origin of 

 fossils could be recorded in plainer language, than by the pheno- 

 menon 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 cir- 

 cumference 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 ; 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. 



