preserved i?t Chalcedony. 521 



hypnum figured at No. 7, have the mammillated appearance whicli 

 implies that they have been formed by the stalagmitic process. And in 

 these the plant is broken, compressed, and deranged, though it pre- 

 serves 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 confervse 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 ba- 

 saltic geodes, the zones always respecting the various parts of the' 

 plant, and forming round it parallel, angular, or curved f^g-ures. 

 From this we may infer that the zoned disposition of those chal- 

 cedonies which go by the name of onyxes, may as well have pro- 

 ceeded by a successive deposition from the centre towards the circum- 

 ference, 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 chalce- 

 dony being thus established, it may be said that it is analogous to 

 the v/ell-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 misht have been the 

 Vol. 11. 3 u 



