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 with agate as many of the previous ones had become filled with green earth. Then an ample infusion of chalcedony pervaded the mass. Numerous doors again opened; some took in a portion of the chalcedony, and then shut; some remained open, and became filled with it; and many more that had been previously filled by the green earth opened their doors again, and the chalcedony pervading the green porous mass, converted it into heliotrope. Then an infusion of lime took place. Doors opened, many of which had been hitherto shut, save for a short time, when the green earth infusion obtained, and became filled with lime; other doors opened for a brief space, and received lime enough to form a few crystals. Last of all, there was a pure quartzose infusion, and doors opened, some for a longer time, some for a shorter, just as on previous occasions. Now, by mechanical means of this character,--by such an arrangement of successive infusions, and such a device of shutting and opening of doors,--the phenomena exhibited by the vesicles could be produced. There is no difficulty in working the problem mechanically, if