NEIGHBOURHOOD OF CORK. 101 



m 



The globular concretions of wavellite are some- 

 times homogeneous, or consist of spicule of the 

 same colour, extending from the centre to the sur- 

 face ; other balls are composed of numerous con- 

 centric layers, distinguishable from one another, by- 

 bands of a different colour, and of different de- 

 grees of transparency. 



Sometimes a small portion only of the wavellite 

 may be observed in the concretion, the remaining 

 part consisting of the matter of the rock, on which 

 (unable to bestow a radiated) the wavellite has iin*- 

 pressed a fibrous fracture *. 



* It appears now to be ascertained, that a certain portion 

 of one substance, having a tendency to crystallize r is able to 

 exert an influence over other substances contiguous, and to im- 

 press upon these its own definite forms. This is very strik- 

 ingly examplified in the case of arragonite ; the crystals of 

 calcareous sandstone from Fountainbleau, and the imperfect 

 crystals of garnets and andalusite in the mica-slate of our own 

 country. 



The specimens sent to the Society, furnish other two very 

 remarkable examples of the same power ; the quartz influencing 

 the calcareous matter, in the construction of the crystals in the 

 limestone j and the matter of the wavellite giving to the clay- 

 slate its own peculiar spherical form, and internal stellular 

 structure. 



It is probable, from these circumstances, that the crystals, 

 which are termed Supposititious, instead of being formed in a 

 space formerly occupied by a true crystal, are in fact true 

 crystals, in which the crystalline substance is concealed in the 

 quantity of foreign matter which it has fashioned,. 



