e vast laboratory. Here was a vesicle filled with green
earth,--there a vesicle filled with calcareous spar,--yonder a vesicle
crusted round on a thin chalcedonic shell with rock-crystal,--in one
cavity an agate had been elaborated, in another a heliotrope, in a third
a milk-white chalcedony, in a fourth a jasper. On what principle, and
under what direction, have results so various taken place in vesicles of
the same rock, that in many instances occur scarce half an inch apart?
Why, for instance, should that vesicle have elaborated only green earth,
and the vesicle separated from it by a partition barely a line in
thickness, have elaborated only chalcedony? Why should this chamber
contain only a quartzose compound of oxygen and silica, and that second
chamber beside it contain only a calcareous compound of lime and
carbonic acid? What law directed infiltrations so diverse to seek out
for themselves vesicles in such close neighborhood, and to keep, in so
many instances, each to his own vesicle? I can but state the
problem,--not solve it. The groups of heliotropes clustered each around
its bulky centrical mass seem to show that the principle of molecular
attraction may be operative in very dense mediae,--in a hard amygdaloidal
trap even; and it seems not improbable, that to this law, which draws
atom to its kindred atom, as clansmen of old used to speed at the
mustering signal to their gathering place, the various chemistry of the
vesicles may owe its variety.

I shall attempt stating the chemical problem furnished by the vesicles
here in a mechanical form. Let us suppose that every vesicle was a
chamber furnished with a door, and that beside every door there watched,
as in the draught doors of our coal-pits, some one to open and shut it,
as circumstances might require. Let us suppose further, that for a
certain time an infusion of green earth pervaded the surrounding mass,
and percolated through it, and that every door was opened to receive a
portion of the infusion. We find that no vesicle wants its coating of
this earthy mineral. The coating received, however, one-half the doors
shut, while the other half remained agap, and filled with green earth
entirely. Next followed a series of alternate infusions of chalcedony,
jasper, and quartz; many doors opened and received some two or three
coatings, that form around the vesicles skull-like shells of agate, and
then shut; a few remained open, and became as entirely occupied w