ROCKS OF THE TRAPP-FOR MATION; U5 



in them in the form of round nodules. The original fluidity of 

 thefe nodules is evinced by their figured and fometimes chryf- 

 tallized ftructure, and indeed is fo generally admitted, that 

 the only queftion refpecling them is, whether this fluidity was 

 trie effect of heat or folution. To anfwer this queftion Dr. 

 Hutton obferves, that the formation of the concentric coats of 

 which the agate is ufually compofed, has evidently proceeded 

 from the circumference toward the center, the exterior coats 

 always impreffing the interior, but never the reverfe. The 

 fame thing alfo follows from this other fact, that where there is 

 any vacuity within the agate, it is ufually at the center, and 

 there too are found the regular chryftals, when any fuch have 

 been formed. It therefore appears certain, that the progrefs and to have con- 

 of confolidation has been from the circumference inwards, and w i tnout: 

 that the outward coats of the agate were the firft to acquire 

 folidity and hardnefs. Now it mull: be confidered that thefe 

 coats are highly confolidated ; that they are of very pure fili- They are pure 

 ceous matter, and are utterly impervious to every fubftance ^^S^!^* 

 which we know of> excepting light and heat. It is plain there- any other tub- 

 fore, that whatever at any time, during the procefs of confoli- ftance 3 

 dation, was contained within the coats already formed, mull 

 have remained there as long as the agate was intire, without 

 the leaft poffibility of efcape. But nothing is found within 

 the coats of the agate fave its own fubftance ; therefore no ex- 

 traneous fubftance, that is to fay no folvent, was ever included 

 within them. The fluidity of the agate was therefore fimple, whence this flu- 

 and unaffifted by any menftruum. In this argument nothing "7 ,s ? e 

 appears to me wanting that is neceifary to the perfection of a fimple, without 

 phyfical, I had almoft faid of a mathematical demonftration. any menftruum* 

 It feems, indeed> to be impoffible that the igneous origin of 

 foffils could be recorded in plainer language, than the pheno- 

 menon which has been juft defcribed.* 



It is well known to geognofts, that during the depofition of Reply. The 

 certain kinds of ftrata, confiderable quantities of air have been ^Hular ftruaure 



. . / ' • of the ftrata 



.formed, and this in endeavouring to efcape has given them a a fcribed togasj 



cellular ftruclure. The almond-ftone (mandelftein) in which 



agates are moft frequently found, has acquired its cellular 



ftructure in this manner. Thefe cells appear to have been the cells are 



fucceffively filled with a ftony folution, out of which thin coats ? ated , t0 have ,.. 

 /•• ■• 111 a •• been fucceluvely 



of jafper, carnehan, chalcedony, &c. have, been precipitated, filled with a 



according as the precipitate altered or was finer. We are na- &° n 7 folution j 



1 2 turally 



