188 ON THE CONCRETIONARY AND 



coats, in consequence of the successive deposition of a 

 material differing in texture or colour. If the agate 

 contains a central portion of calcareous spar, it is 

 obviously only a variation of the former case. It is 

 thus also easy to explain, why the agate sometimes 

 contains an interior covering of siliceous crystals, 

 from changes that have taken place in the quality of 

 the solution ; these presenting their usual geometric 

 forms, or else being confused, accordingly as the 

 cavity is filled or not. 



It cannot be objected that siliceous earth is insoluble 

 in water ; because its solubility is proved by numerous 

 facts, and by none more decidedly than the existence 

 of vegetable remains in chalcedony. And that the 

 solid substances in question can transmit water, is 

 certain ; since it exists in rocks, and finds a passage 

 through many much more solid than the amygdaloidal 

 bases, as is proved by the daily formation of calca- 

 reous stalactites. I have also proved, that the agates 

 are sufficiently porous to transmit oil, and also sul- 

 phuric acid ; that property being the basis of the 

 process used for staining them black. There is there- 

 fore no difficulty in understanding, 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 for the amygdaloidal nodules 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 



