and the allied Rocks. 53 



some instances by infiltration ? They are the very places where 

 an infiltrating fluid would deposit its sediment, or collect and 

 crystallize, if capable of crystallization ; and such infiltrating 

 fluids are known to permeate all rocks, even the most solid, 

 and especially if beneath a body of water. It is evident, there- 

 fore, that we are supporting no strange or improbable hypo- 

 thesis. On some volcanic shores one variety of the process 

 may be seen in action. The cavities of a lava may be de- 

 tected in the process of being filled with lime from the sea- 

 water washing over dead shells or coral sand, and at times a 

 perfect amygdaloid is formed. But the positions and cha- 

 racters of the minerals themselves establish clearly the view 

 we support. 



2. The mineral in these cavities sometimes only fills their 

 lower half, as if deposited from a solution ; and again, it in- 

 crusts the upper half or roof, as if solidified on infiltrating 

 through. In the large geodes of chalcedony, stalactites de- 

 pend from above like those of lime from the roof of caverns, 

 and, as Macculloch states, the stalactite is often found to cor- 

 respond to an inferior stalagmite,the fluid silica having dripped 

 to the bottom and there become solid ; moreover, the superior 

 pendent stalactite is sometimes found united with the stalag- 

 mite below. The same results are here observed as with lime 

 stalactites in caverns, and often a similar laminated or banded 

 structure, the result of deposition in successive layers. Such 

 results can proceed only from a slow and quiet process, — a 

 gradual infiltration of a solution from above into a ready- 

 formed cavity ; they can no more be supposed to arise from 

 ascending vapours, or gaseous emanations from below, than 

 the stalactite in the limestone cavern. 



Another fact is often observed. A geode of quartz crystals, 

 sometimes amethystine, in which every crystal is neatly and 

 regularly formed, is found with the surface coated over with 

 an incrustation of chalcedony, the part above hanging in small 

 stalactites; and this chalcedonic coat sometimes scarcely ad- 

 heres to the crystals it covers ; or is even loose, and may be 

 easily separated. There can scarcely be a doubt of a subse- 

 quent infiltration in a case of this nature. 



We might rest our argument here, since the fact being as- 

 certained with regard to quartz, it is necessarily established as 

 a general principle with reference to the zeolites and other 

 amygdaloidal minerals ; for quartz or chalcedony, when pre- 

 sent in these cavities, is, with rare exceptions, the lower or 

 oilier mineral. We find zeolites implanted on quartz, but 

 very seldom quartz on zeolites. I have met with no instance 

 of the latter, while the former is the usual mode of occurrence. 



