321 



agatine and other silicious nodules may have been formed, to the en- 

 tire exclusion of the solvent. 



But this assumed process being even allowed, it must be ad- 

 mitted, that the following variations of it must be liable to occur. 

 The aperture, by which the cavity is supplied, may be closed by the 

 crystals at its sides meeting, previously to the filling of the cavity 

 by crystallizations : or, instead of the liquid being admitted by one 

 aperture, it may percolate through numerous small openings into 

 the cavity. It is therefore necessary to see how the process will be 

 affected by these variations. 



The aperture may become closed, under two circumstances : there 

 may, or there may not, exist some small fissures, or pores, by which 

 the fluid remaining in the cavity, after the supply has failed, may 

 drain off. If the former be the case, the hollow, empty, crystalline 

 nodule remains ; and if the latter, the water is inclosed, and the 

 enhydros, so often mentioned by lithologists, is formed. 



It now remains to inquire, what would be the consequence of 

 the fluid infiltrating, through the substance forming the top and 

 sides of the cavity, instead of entering by one aperture only. Here, 

 as in the other instances, condensation would commence at the cir- 

 cumference, or sides of the cavity ; but would go on so slowly, as 

 to admit the further infiltration of the fluid, tor a considerable time ; 

 and the same porosity of the substance forming the bottom of the 

 mass, would likewise allow the gradual exit of the superfluous fluid. 

 The process which is thus carried on, in this small cavity, will be 

 found to agree, almost in every respect, with that, which we shall 

 soon see has taken place, in caverns adorned with calcareous 

 stalactites. As in those, so the surrounding substance in these 

 smaller cavities differs, in closeness of texture, in different parts; 

 opposing more resistance to the infiltration of the fluid, in some 

 parts than in others : hence the fluid oozes in a partial and irregular 

 manner, according to the resistance it meets with. In some parts, 



VOL. I. T T 



