Formation of Flints in Chalk. 419 



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 suc- 

 cessively 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 con- 

 solidation 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 intirely 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 be offered, to explain the formation of quartz crys- 

 tals 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 laminae are disposed 

 in parallel lines, crossing horizontally the cavity in which they are contained, and some- 

 times 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 ex- 

 actly 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 laminas 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 posi- 

 tion of their parallel plates, they lie always horiaoatally. 



