Crystalline Structures of Rocks. 77 



stalagmite; offering a case precisely resembling that wliich occurs 

 in the ordinary calcareous stalactites of caverns. Lastly, the de- 

 pendant stalactite is more or less perfectly imbedded in a laminar 

 chalcedony, rising from the bottom of the cavity till it is at last des- 

 tined to fill it, and thus to form a solid nodule. If any appear- 

 ances can prove a watery infiltration of silicious matter, these are 

 of that nature. In other instances, the silicious stalactite is invol- 

 ved in calcareous spar, which, as in the former case, either leaves 

 an empty space or fills the whole ; forming a compound amygda- 

 loidal nodule. Here it is evident that the calcareous spar is pos- 

 terior to the stalactite ; and thus also a watery infiltration of two 

 minerals into one cavity is proved. 



It is easy to extend this reasoning to the ordinary case of the 

 concentric agate nodules, which may or may not contain calcareous 

 spar. In these cases, the siliceous matter has been deposited by 

 a more gradual infiltration over the whole of the surface of the air 

 vesicle ; producing the concentric appearance of the coats, in con- 

 sequence of the successive deposition of a material diifering in 

 texture or colour. If the agate contains a central portion of calca- 

 reous 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 must not be objected to this explanation, that siliceous earth is 

 insoluble in water ; because that is proved by numerous facts, and 

 by none more decidedly than the existence of vegetable remains in 

 chalcedony. Nor must it be said that the solid substances in 

 question cannot transmit water. Water is known to exist in rocks, 

 even in the traps, and to find a passage through many, much more 

 solid than the araygdaloidal bases, as is pioved by the daily forma- 

 tion of calcareous stalactites. I have also shown, in a paper in the 

 Edinburgh Journal, that the agates are sufficiently porous to trans- 

 mit oil, and also sulphuric acid ; that property being the basis of 

 the process used for staining them black. There is therefore no 



