54 On the Minerals of Trap and the allied Rocks. 
2. The mineral in these cavities sometimes only fills their lower 
half, as if deposited from a solution; and again, it incrusts the 
upper half or roof, as if solidified on infiltrating through. In the 
large geodes of chalcedony, stalactites depend from above like 
those of lime from the roof of caverns, and, as Macculloch states, 
the stalactite is often found tocorrespond 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 stalagmite 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 succes- 
sive layers. Such results can proceed only from a slow and 
quiet process,—a gradual infiltration of a solution from above into 
aready formed cavity; they cannot be supposed to arise from 
ascending vapors, or gaseous emanations from below, no more 
than the stalactite in the limestone cavern. 
Another fact is often observed. A geode of quartz consti 
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 sta- 
lactites; and this chalcedonic coat sometimes scarcely adheres to 
the crystals it covers,—or is even loose and may be easily sepa- 
rated. There can scarcely be a doubt of a subsequent infiltration 
ina case of this nature. 
We might rest our argument here, since the fact being ascer- 
tained with regard to quartz, it is necessarily established as a 
general principle with reference to the zeolites and other amyg- 
daloidal minerals: for quartz or chalcedony, when present in these 
cavities, is, with rare exceptions, the lower or outer mineral. We 
find zeolites implanted on quartz, but very seldom quartz on zeo- 
lites. I have met with no instance of the latter, while the former 
is the usual mode of occurrence. Any deduction, therefore, re- 
specting quartz, holds equally for the associated minerals. 
How a cavity coated with a deposit of chalcedony can still be 
afterwards filled up with other minerals, has been deemed a mys- 
tery in science, but the possibility of itis now not doubted. . Kven 
flint and agate, as Macculloch states, are known to give passage 
to oil and sulphuric acid; and much more will this take place in 
the moist rocks before the agate has been hardened by exposure 
to the air. Silica remains in a gelatinous state for a long period 
