6 Mr. Toulmin Smith on the Formation 



found with spines affixed, and therefore aUve or with unde- 

 composed soft parts when entombed. The masses of flint to 

 which they are affixed are very frequently not attached to either 

 of the large orifices of the shell, but to some part of the sides, 

 while the shell is entirely filled with flint and both orifices closed. 

 Mr. Bowerbank states that, when the shell is not entirely filled 

 with flint, in " the space thus unoccupied by the flint was always 

 included one or both of the large orifices of the shell*." I do 

 not find this fact in any degree borne out by my own observations. 

 I have specimens at this moment before me in which the reverse 

 is the fact, — both orifices being closed though the Echinite has 

 never been filled with flint. Many other illustrations of the clear 

 inapplicability of the sponge theory to the case of Echinites might 

 be given. 



It is frequently the case that the remains of a zoophyte are 

 found in the exact centre of a flint which is externally round. In 

 some localities these abound, and they occur from the condition 

 of a very friable substance, perfectly preserving the structure 

 however, and merely coated with a layer of flint, to the same 

 body perfectly solid and silicified throughout, though still clearly 

 showing the structure. It is easy to understand that, in parti- 

 cular classes of objects (such e. g. as some of the beautiful ob- 

 jects inclosed in the Wiltshire flints) and under particular cir- 

 cumstances, the affinity for the siliceous fluid might not be suf- 

 ficiently great, or the rapidity of its solidifying too great, to 

 allow of its penetrating the body round which it formed. In such 

 cases we should necessarily find, as we do, that the soft animal 

 substance, not being penetrated, has decayed f, leaving only what 

 we find, — the hard and, so protected, indestructible parts ; where 

 it was wholly soft animal matter a mere hollow is found. The ex- 

 tent to which penetration extended will of course afi'ect the facts 

 exhibited. The explanation offered by Mr. Bowerbank of the 

 Wiltshire flints is clearly inadmissible. Had they been dead and 

 sponges grown over them, it is obvious that they would not, as 

 now, have been loose within the flints, but solidly encased, as 

 shells and corals are found. 



I have an Asterias in the centre of a flint. Teeth, pieces of 

 wood, &c. are often found in flints, which could never have be- 

 come entangled within the meshes of a sponge. It will be said 

 that the sponge may have been built over them. This, though 

 highly improbable, may be possible ;. but there are cases to which 

 such an explanation cannot apply, in which flint is found in situ- 



• Loc. cit. p. 1 86. 



f I believe it may be stated as an invariable rule, that there is an orifice, 

 large or minute, in all such cases, through which the gases evolved might 

 escape. 



