Extract

Chalcedonic and zeolitic druses represent the former vapour vesicles of igneous rocks, which vesicles have, after partial alteration of the latter, been more or less completely filled by the deposition of decomposition-products of the several minerals which compose the rocks. These decomposition-products have been carried in solution—probably in water—into the vesicles by endosmose—as into shut cavities. Within the vesicles they have been deposited in successive layers, through which layers endosmose continued to act. After the deposition or coagulation of the material forming the layers, the solvent liquid was forced out of the cavity through one or through many openings by the entrance of a new quantity of strong solution, according to the ordinary law of endosmose. These openings may be called tubes of escape.

When the druse has been more or less completely filled by the various modifications of silica—hydrated or anhydrous—it is termed an agate, but no absolute line can be drawn to separate such from a zeolitic cavity, as one cavity may contain both siliceous layers and zeolites, and that in varying order of superposition. Neither can it be definitely stated which ingredients of the rock by its decomposition determines the formation of either; but this distinction has to be pointed out in the formation or filling-up of such druses, namely, that there is a perfectly definite order in the successive deposition of the various zeolitic minerals which fill drusy cavities, while there is no definite order whatsoever in the deposition of the various forms of silica, the

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