CRYSTALLINE STRUCTURES OF ROCKS. 187 



origin to inflation. It is then into previous cavities 

 that the minerals of the amygdaloids have been de- 

 posited ; and it only remains to inquire whether this 

 has been effected during the igneous condition of the 

 rock, or from posterior infiltrations of a watery solu- 

 tion of earths. It must not here be objected, that the 

 larger cavities could not have been produced by infla- 

 tion ; for it is in those, more particularly, that the 

 proofs of watery infiltration are most satisfactory. 



I have shown, in the account of the Western 

 Islands and elsewhere, that stalactites of chalcedony 

 were often found to depend from the upper parts of 

 such cavities, partly filling the vacuity. In other 

 cases, the stalactite is found to correspond with an 

 inferior stalagmite ; offering a case precisely resem- 

 bling that which occurs in the ordinary calcareous 

 stalactites of caverns. Lastly, the superior dependent 

 stalactite is more or less perfectly imbedded in a 

 laminar chalcedony, rising from the bottom of the 

 cavity which it is at last destined to fill, and thus to 

 form a solid nodule. If any appearances can prove a 

 watery infiltration of siliceous matter, these are of that 

 nature. In other instances, the siliceous stalactite is 

 involved in calcareous spar, which, as in the former 

 case, either leaves an empty space or fills the whole ; 

 forming a compound amygdaloidal nodule. Here, it 

 is evident that the calcareous spar is posterior 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 case& 5 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 



