Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/531

Rh hypnum figured at No. 7, have the mammillated appearance which implies that they have been formed by the stalagmitic process. And in these the plant is broken, compressed, and deranged, though it preserves its natural colour. In many others no mechanical texture or disposition can be discovered in the stone, but the whole appears one semitransparent mass of chalcedony. These are the specimens in which the confervæ preserve that freedom of arrangement and that perfection both of colour and structure which seem to imply that they had been so suddenly involved in a mass of siliceous matter as to have been preserved from all future changes. Many other specimens have that mixed aspect of jasper and chalcedony to which among other varieties of ornamental stones the lax term of agate has been, applied. In some specimens we may observe that zoned appearance which so generally characterizes the chalcedonies occupying the basaltic geodes, the zones always respecting the various parts of the plant, and forming round it parallel, angular, or curved figures. From this we may infer that the zoned disposition of those chalcedonies which go by the name of onyxes, may as well have proceeded by a successive deposition from the centre towards the circumference, as in the reverse order, an arrangement supposed by some mineralogists to have been the cause of this structure in the nodules found in trap.

The fact of the existence of entire vegetable remains in chalcedony being thus established, it may be said that it is analogous to the well-known instances of silicified wood, and of animal remains similarly situated. Yet it offers some important differences which may throw light on certain disputed points, and lead to conclusions of no small consequence, conclusions not so universally resulting from those facts. It has been maintained on one hand that the silicification of wood and of animal remains might have been the

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