200 Mr J. D. Dana on the Ori(/in of Trap Minerals. 



times a perfect amygdaloid is formed. But the positions and 

 characters 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 

 correspond to an inferior stalagmite, the fluid silica having 

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

 superior pendent stalactite is somethnes found united with 

 the stalagmite below. The same results are here observed 

 as with lime stalactites in caverns, and often a similar lami- 

 nated or banded structure, the result of deposition in succes- 

 sive 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 cannot be supposed to arise 

 from ascending vapours, or gaseous emanations from below, 

 no more 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 

 adheres to the crystals it covers, — or is even loose, and may 

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

 sequent 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 

 outer 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. 

 Any deduction, therefore, respecting quartz, holds equally 

 for the associated minerals. 



How a cavity coated with a deposit of chalcedony can still 



