1880 .] 
97 
[Wadsworth. 
by the segregating powers of crystallization ; and in illustration of the pro- 
cess we are pointed to the many segregations of feldspar, quartz, and mica, 
in granite and other rocks, the siliceous nodules in many sandstones, the 
pearlstones in trachytes and obsidian. Others have thought them foreign 
pebbles, enclosed at the time the rock was formed. Again, they are de- 
scribed as proceeding from the vapors which permeated the rock while still 
liquid, and which condensed as the rock cooled, in cavities produced 
by the vapors . By a few it is urged, admitting that the cavities are 
inflations by vapors like those of common lava, that they may have been 
filled either at the time the rock cooled or at some subsequent time, either 
by crystallization from vapors, or from infiltrating fluids, but more generally 
the latter. Of these views we believe the last to accord best with the 
facts. * * * * * * The cavities occupied by the nodules are in every 
respect similar to the common inflations or air bubbles in lava. These cav- 
ities are open and unoccupied in common lava, and may be no less fre- 
quently so in the ejections under water; and should they not be expected 
to fill in some instances by infiltration ? They are the very places where an 
infiltrating fluid would deposit its sediments, 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. 
* * * * * 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 chal- 
cedony, stalactites depend from above like those of lime from the roof of 
caverns, afld, 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 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 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 cannot be supposed to 
arise from ascending vapors, or gaseous emanations from below, no more 
than the stalactite in the limestone cavern. ******** The zeolites 
occupy veins or seams as well as cavities. Often the seams were opened 
by the contraction of the cooling rock, and at other times they were of more 
recent origin. In either case the minerals filling these seams must be sub- 
sequent in formation to the origin of the rock itself, and could not have pro- 
ceeded from vapors attending the eruption. These seams sometimes open 
upward and can be seen to have no connection with the parts below, the rock 
in this portion being solid. Origin from above or from either side, is the only 
supposition in such cases. * * * * Zeolites, moreover, have been found 
PROCEEDINGS B. S. N. H. VOL. XXI. 7 SEPTEMBER, 1881 . 
