of the Chalk. 83 



times mere mechanical action was sufficient to induce solidifica- 

 tion. I further showed* that from these facts certain very re- 

 markable and hitherto unnoticed results had followed ; namely, 

 that when a mass of the siliceous fluid had solidified thus sud- 

 denly round a soft body, a Ventriculite for example, in which the 

 soft parts were existing at the time of its envelopment, it neces- 

 sarily formed a solid mould round the nucleus : that when the 

 soft body decayed, its gases and softer components having escaped 

 through or been absorbed by the surrounding and hardening 

 chalk, a more or less complete hollow was necessarily left in the 

 solid flint in the place formerly filled by the animal body : that 

 the firmer fibre of such bodies remaining after the softer parts 

 had thus passed away would afford and did afford centres of 

 crystallization for any silex slowly permeating the stratum in a 

 liquid or gaseous form after that stratum had acquired consistency: 

 that according to the greater or less quantity of such permeating 

 silex would be the result in either wholly or only partially filling 

 up the hollows whose origin I thus explained. I showed the ac- 

 cordance of these views with the known laws of the development 

 of crystals f. 



The most important results follow from these observations ; 

 and I immediately saw that that which had been generally de- 

 scribed as the remains of the body itself was in truth the incrus- 

 tation of a crystallized foreign substance round such remains. It 

 will be obvious how erroneous must be all descriptions founded 

 upon the former notion. 



It remained to adopt a mode of examination of the Ventricu- 

 lidse in chalk and in flint which should realize the living animal in 

 the same form from an inspection of its remains in either matrix. 

 To effect this I made many sections of chalk specimens in every 

 direction, for examination both as opake and transparent objects, 

 — an attempt which I believe was novel. Aware that it is by 



* Ann. and Mag. Nat. Hist. vol. xix. p. 306. 



f The mode of crystallization most commonly exhibited in these cases is 

 the acicnlar. Silex is known to crystallize in this way from sublimation. In 

 the 2nd vol. of the 'Transactions of the Geol. Soc' p. 274, is a paper by 

 Dr. MacCulloch on an instance observed by him of the actual formation of 

 such crystals, which he there describes aptly as " filamentous crystals cross- 

 ing each other in all directions." It seems to me, from careful observa- 

 tion, that it was in cases where an almost or altogether entire hollow was 

 left, all the fibre having decayed as well as the soft parts, that the geometric 

 crystals were more disposed to form within the flints. The centres were 

 fewer, the space larger. A. botryoidal form is frequently assumed under such 

 circumstances by the aggregation of acicular crystals ; and traces of its former 

 existence are visible in many now solidly filled spaces. The greater or less 

 degree of slowness of the deposit no doubt had an important influence in 

 determining the silex to assume the geometric form of so-called quartz or 

 the botryoidal form of so-called calcedony ; the quartz being the result of a 

 still slower deposit than the calcedony. 



