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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
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