Page:Transactions of the Geological Society, 1st series, vol. 4.djvu/429

 a reddish chalcedony. The introduction of this chalcedony appears to have been subsequent to the first incasing of the body by the coarse siliceous crust, and contemporaneous with the gradual decay of the animal matter inclosed, the particles of chalcedony being successively introduced into the space vacated by the animal particles as they successively perished, till the result was an entire substitution of chalcedony bearing the form of the organization of the animal. Although in the present compact state of the matter of flint it is not easy, though possible, to force a fluid slowly through its pores, it is probable that before its consolidation was complete it was permeable to a fluid whose particles were finer than its own, and that the particles of chalcedony, whilst yet in a fluid state, being finer than those of common flint, did thus pass through the outer crust to the inner station they now occupy, where they also allowed a passage through their own interstices to the still purer siliceous matter which is often crystallized in the form of quartz in the centre of the chalcedony, and so entirely surrounded by it, that it could have had no access to its present place, except through the substance of the chalcedony and flint inclosing it.

Perhaps the same illustration may he offered, to explain the formation of quartz crystals in the centre of many agates, as well as of their concentric chalcedonic zones, the substance of which appears often to increase in purity in proportion to its distance from the outer circumference. I allude particularly to those agates in which there are no traces of any funnel through which the matter of the concentric zones could have been introduced; and to those chalcedonic geodes in the basalt of the Giant's Causeway, in which also no sign of a funnel can be discovered, but the component laminæ are disposed in parallel lines, crossing horizontally the cavity in which they are contained, and sometimes filling only the lower region of it. In such cases, the upper and void portion of the cavity is lined with an uniform thin film or arch of mammillated chalcedony, so exactly conformable to the irregularities of the hollow within which it is deposited, that we can only suppose it to have been introduced by a slow and uniform infiltration through every pore of the cavity that is now lined by it, and which, had the process been continued further, might intirely have filled it up.

This seems indeed to have happened in the case of those solid geodes, of which the lower part is composed of parallel flat plates of chalcedony, and the upper part made up of curved zones of the same substance concentric with each other, and bearing the form of the arch that overhangs the horizontal laminæ of the lower region.

Those geodes in which the cavity of the upper region is open, and merely lined by a thin vaulting of chalcedony, are known at the Giant's Causeway by the appellation of Box Agates, and Dr. MacDonnel has assured me, that in countless instances, when he has broken the laminated geodes from their matrix, with a view to examine the position of their parallel plates, they lie always horizontally.