SPONGES IN FLINT. 213 



driven slightly inwards into the body of the flint, and the 

 result is that a circular fissure is produced between that 

 part of the flint which is condensed for 

 the moment by the blow, and that part 

 which is left untouched. As each particle 

 in the small circle on which the hammer 

 impinges may be considered to rest on 



c f?i" '~PT?r T more than one other particle.it is evident 

 ficial Cone or .b lint. , ., . , _ ^ ' _ n 



(From Sir J. that the circular fissure, as it descends 

 Evans.) into the body of the flint, will have a 



tendency to enlarge in diameter, so that 

 the piece of flint it includes will be of conical form, the 

 small circle struck by the hammer forming the slightly 

 truncated apex." A little practice will enable anyone to 

 make these flint cones with ease. The size of the cone, 

 and the degree of steepness of its sides, vary with the 

 nature of the flint, the weight and form of the hammer, 

 and the force of the blow. 



When examined with a lens or microscope, chalk-flint 

 frequently exhibits a perfectly uniform structure through- 

 out, without the least trace of the presence of any organic 

 body. In other cases, however, traces of sponges, corals, 

 shells, echinoderms, diatoms, &c., are more or less apparent 

 in flint. Perhaps the most common of all these organisms 

 are the mushroom- shaped sponges known as ventriculites ; 

 and if we examine a flint containing one of these sponges 

 we shall frequently observe that there is a complete 

 transition from portions of the perfectly preserved sponge 

 to homogeneous flint, without the least trace of organic 

 structure. In the case of echinoderms, we shall find that 

 while in some cases the whole interior of the shell (or, as 

 it is technically called, test) is filled with flint, the shell 

 itself retaining its original calcareous structure, in other 

 instances the original shell itself has been completely 

 removed and replaced by flint. In some cases the original 

 structure of the shell has been preserved in the flint, but 

 more generally this has been completely lost, and the flint 

 is structureless. 



This replacement of a calcareous by a siliceous structure 

 is an instance of pseudomorphism. Similar remarks will 



