Flints of the Ujyper or J]liite Chalk. 191 



phery of the clierty wall, the adhesion of the cretaceous 

 particles being due solely to their having become imbedded 

 here and there, while the flinty mass was yet in an unconso- 

 lidated state, into little pits or cavities formed by the pressure 

 of the cretaceous particles themselves, or into other equally 

 accidental irregularities of the surface of the nascent nodule. 

 The adherence of any. portion of chalk to the nodule is a mere 

 mechanical adherence arising out of the grip thus secured. 



Now, according to the replacement theory, the entire mass 

 of black flint was at one time a mass of calcareous ooze, 

 which, becoming impregnated with a fluid aqueous solution 

 of silica, became gradually silicified. Had this really taken 

 place, one of two things must have happened : either the re- 

 placement of the calcareous material must have begun from 

 some central point or points, by the admission of the siliceous 

 solution into the centre of the mass through some channels 

 which communicated with the surrounding medium — in which 

 case a period must have arrived at which the external layer 

 of the mass undergoing silicification must also have come 

 under the influence of the replacing siliceous fluid and have 

 in its turn become completely converted into black flint, the 

 replacement thus extending radially from the centre of the 

 mass to its periphery ; or the replacement must have taken 

 place from without and extended centrewards. It will be 

 obvious, however, that under the latter supposition the outer 

 coating must of necessity have been the first portion of the 

 mass to be converted into flint. But it is almost needless to 

 observe that in neither case is the theory of replacement 

 borne out, inasmuch as in the first-named case the outer 

 coating must sometimes, at least, have been converted into 

 black flint — a condition in which it is never found ; and in 

 the second case, the silicification having begun from the peri- 

 phery towards the centre, nodules must occasionally have 

 been met with in which a coating of black flint {iiot possess- 

 ing a cherty external layer) surrounded the yet unmetamor- 

 phosed central calcareous mass — another condition in which 

 we never find it unless under the wholly exceptional circum- 

 stances where the nodule has, after separation from its chalky 

 matrix, undergone attrition. 



In the early nascent state of each nodule no chemical 

 replacement of mineral for mineral has taken place on either 

 side — the extensions of the colloidal siliceous jelly, and the 

 intervening masses of calcareous deposit interlacing mechani- 

 cally, and changing their relative boundaries only in obedi- 

 ence to the slow contraction going on in the colloidal mass 

 towards its own centre or a centre or point d^appui consisting 



14* 



