78 Dr. Mac Culloch on the Concretionary and 



difficulty in understanding, both how the rocks should admit the 

 mineral solutions into their cavities, and how the first crust of agate 

 should permit the deposition, not only of successive ones of the 

 same nature, but, from changes in the nature of the solution, of 

 calcareous spar also. 



One source of the amygdaloidal nodule is thus established, but 

 it does not follow that this is the sole one. The minerals which 

 these cavities contain are numerous and various, and we have no 

 proof that some of them can be formed by aqueous deposition ; 

 while it is certain that they are sometimes produced from fusion, 

 as they are found constituting imbedded parts of the volcanic 

 rocks. I have shewn in the Geological Transactions, that silica can 

 be sublimed by heat ; and the same fact has been affirmed to 

 occur in the volcanic products of Vesuvius, by observers whose 

 testimony cannot be questioned. It is possible that compound 

 minerals may be subjected to the same laws ; and it is also per- 

 fectly intelligible, how in a fluid or tenacious rock containing the 

 cavities produced by inflation, those minerals which have some- 

 times crystallized in the general mass, should have also protruded 

 themselves into the cavities. 



There are probably, therefore, two origins to be assigned to the 

 amygdaloidal nodules, both of the trap rocks and the volcanic 

 products ; however the mode of explaining the igneous method 

 may here differ from that adopted by Dr. Hutton. Admitting them 

 both, the question respecting the igneous origin of the amygda- 

 loidal bases of the trap rocks, rests precisely on the same founda- 

 tion as before ; as the essential circumstance consists, not in the 

 presence of the nodule, but in the formation of the cavity that 

 contains it. 



Of the Nature of the Co?icr€tionan/ Structure. 



Having thus terminated all which it appeared requisite to say 

 respecting several varieties of concretionary structure, it remains 

 to see if any light can be thrown on the general nature of this mys- 

 terious process. 



That it differs essentially from crystallization, was already no- 



