October 1, 1892.] 



KNOWLEDGE 



197 



subsequently gained its present condition, or whether it 

 was added at certain intervals during the deposition of the 

 chalk from a totally different source, or was introduced 

 subsequently to the deposition of the whole. 



I believe 1 am right in saying that there is still a current 

 notion among many persons who are not scientific geologists 

 that dint is a kind of igneous rock ; and its hardness and 

 superficial resemblance to some kinds of obsidian may at 

 first sight lend some countenance to such an idea. Never- 

 theless, it is almost superfluous to add, such a notion is a 

 totally erroneous one, and the mode in which flint occurs — 

 without altermg the limestone with which it is in contact, 

 or the fossils which it contains — is of itself suflicient to 

 refute any such origin. 



From the hardness and insolubility of flint, it would 

 appear a very natm-al inference that silica in all forms is 

 likewise insoluble, but, as proved by siliceous springs, 

 silica in a certain condition is freely soluble in alkaline 

 waters. This naturally suggested to the earlier geologists 

 that flints had been deposited by the aid of hot springs or 

 heated waters in the cretaceous sea at certain intervals 

 during the formation of the chalk sea. Thus, the late 

 Dr. Mantell wrote that " the nodules and veins of flints 

 that are so abundant in the upper chalk have probably 

 been produced by the agency of heated waters holding 

 sdex in solution, and depositing it when poured into the 

 chalk sea." It will, however, be obvious that there are 

 very serious difliculties in accepting any such explanation. 

 In the fii-st place, we should require the introduction of 

 floods of heated water containing silica at certain irregular 

 intervals during the whole period of the deposition of the 

 upper chalk : and it would also be essential that these waters 

 should have extended over vast areas of ocean. Secondly, 

 the mode in which nodular flint occurs could not 

 adequately be explained by any theory of this kind ; while 

 it would leave the origin of the tabular flint, which we have 

 seen may occur in joints and fissures, totally unaccounted 

 for. Another idea was that the silica had been, at least 

 partially, introduced from above after the deposition of the 

 chalk; but, although this explanation may, and perhaps 

 does, partly account for the formation of some of the 

 upper tabular flints, it is quita impossible that it could 

 explain the formation of the numerous layers of nodular 

 flints throughout the body of the chalk. 



There is, however, an explanation which will readily 

 account for all the features presented by the chalk-flints, 

 and requires the aid of no foreign factors in the process. 

 This explanation is based on the phenomenon known as 

 segregation. Now segregation is the tendency presented 

 by a small quantity of one substance, when diffused 

 through a much larger quantity of another substance, to 

 collect together in nodules or strings, which generally 

 accumulate either around some fragment of their own 

 nature or some foreign body — especially an organic one — 

 as a nucleus. We have well-known examples of this 

 segregating process in the huge lenticular calcareous masses 

 termed " septaria," found in the London and Kimeridge 

 clays, and also in the iron-nodules of other formations. 

 Premising that soluble silica has a peculiar affinity not 

 only for any kind of silica, but likewise for gelatinous 

 organic substances (both of which were presented by the 

 sponges of the cretaceous seasj, it will be obvious that if 

 we can only satisfy ourselves that at the time of its 

 deposition the chalk contamed diffused among its substance 

 a sufiieient amount of soluble silica, we shall at once be able 

 to account for the formation of its flint. Now, as we pass 

 upwards in the cretaceous system from the lower green- 

 sand to the upper chalk, we find a gradual change fi'om 

 a completely siliceous to a calcareous rock. Moreover, 



while in the gault, upper greensand, and lower chalk (in 

 which in the south of England there are no flints) tliere 

 is a large but decreasing amount of soluble silica, varying 

 from 40 per cent, in the upper greensand to 31 per cent, in 

 the chalk-marl, when we reach the upper chalk with 

 flints such soluble silica is reduced to a mere trace. As 

 it is at the base of the white chalk that the sponges attain 

 their greatest development, and as it is also here that 

 flints first commence, the disappearance of the soluble 

 silica may be safely attributed to its segregation by means 

 of the sponges and other bodies, and its conversion into 

 dint. Prof. Prestwich, writing on this subject, observes 

 that in presence of the siliceous spicules of the cretaceous 

 sponges and thek gelatinous animal matter, " the soluble 

 or colloidal silica, dispersed through the soft chalk-mud, 

 slowly segregated from out of the surrouudmg pulpy mass, 

 and gradually replaced part or the whole of the organic 

 matter, as it decayed away. Nor has it stopped there ; 

 owing to the affinity of the particles of colloidal silica 

 amongst themselves, the segregation has not ceased with 

 the replacement of the organic body, but has continued so 

 long as any portion of silica remained in the surrounding 

 soft matrix ; whence the frequent excess of flint beyond the 

 interior or body of the shells, echinoderms, itc, and whence 

 also the irregular shape arising from this overgrowth of the 

 flint nodules.'' Next to sponges, echinoderms seem to 

 have afforded the most attractive centres of segi'egation ; 

 and while in some cases only then- shells have been filled 

 with flint, in other instances we find a mass of these shells 

 cemented together by a nodule of flint. 



In some parts of the Continent, and also in Yorkshire, 

 we find that for some reason or other — not improbably a 

 greater development of sponges and a smaller amount of 

 soluble sihca — the segregating process has extended down- 

 wards to the lower chalk, where flmts are then found ; and 

 we suspect that in such cases analysis would also show in 

 these beds a corresponding absence of free soluble silica. 



With regard to the so-called "potstones" of the 

 Norfolk chalk, some of which may be upwards of a yard 

 in height, with a diameter of a foot or so, the only adequate 

 explanation of their formation that has yet been offered is 

 that they represent gigantic cup-like sponges which have 

 grown one upon the top of the other, as they were succes- 

 sively buried in the newly-formed chalk, and that they 

 have been subsequently silicitied by the same segregating 

 process. 



We conclude, therefore, that the flints of the chalk were 

 originally an integral portion of the rock itself, which was 

 then a slightly sihcated limestone ; and that the present 

 purely calcareous character of the chalk is due to the 

 separation of the silica by segregation. We have, however, 

 still to account for the relatively large amount of soluble 

 silica present in the cretaceous rocks, since this is far in 

 excess of what would have been brought down by most 

 rivers of the present day, in the waters of which the 

 amoimt of this substance is almost infinitesimal. It has 

 been suggested that the unusual supply may have been 

 afforded by the cretaceous rivers being largely fed by 

 siliceous springs ; but although this may have been one 

 factor in the case, a more probable theory is that the 

 drainage area supplying the cretaceous sea with sediment 

 was largely composed of decomposed felspathic rocks, in 

 which the amoimt of this silica would have been amply 

 sufficient to have furnished the quantity present in the 

 chalk. 



It is, however, not only with regard to its mode of 

 origin that flint is of more than ordinary interest. Being 

 an excessively hard substance, it is one exceedingly diffi- 

 cult to be worn to powder by the action of water, and the 



