392 E. M. BUR WASH CHALK AXD FLINTS OF NORTHERN FRANCE 



evidence against the supposition of the solution of large quantities of 

 them in the chalk and their redeposition as chalcedony. They consist of 

 colloidal silica, which is not readily soluble in the presence of electrolytes 

 such as dissolved chalk, salt from sea-water, and the products of their 

 reaction (2^aCl + CaCOg X ^aaCOg + CaClg). Insoluble silica, on 

 the other hand, as well as silicates such as cla}^, are rendered soluble by 

 the presence of NagCOg, and may be redeposited as colloidal silicic acid 

 by treatment with HCl, which is probably present in the fissures, as 

 shown elsewhere in this paper. This colloidal silica as it dries changes 

 slowly back to insoluble silica, and if we suppose this process to have 

 proceeded slowly to partial completion, giving time for crystallization of 

 the silica, the chalcedonic condition of the flints, formed partly of fibrous 

 crystals and to a less extent of colloidal silica, is satisfactorily explained. 

 This also explains the presence in flints of small quantities of alumina, 

 magnesia, and iron and the slight dolomitization of the included chalk. 

 These circulating waters were, therefore, undoubtedly responsible for 

 the deposition of the flint nodules which occur so abundantly along the 

 bedding planes and also between them, but almost invariably along the 

 joints or other fissures, so far as the writer's observation goes and as 

 English authorities agree. 



Theory of Formation of Flint Xodules 



The theory has been held that these flint nodules were due to concre- 

 tionary or gravitational aggregation of siliceous sponge spicules, radiola- 

 rian tests, or other siliceous matters deposited at the same time as the 

 chalk or alternately with it;* but this theory seems quite doubtful for 

 several reasons : 



(1) The platelike and roughly cylindrical shapes of many flints show 

 that the}' have been deposited in enlarged joint cracks or in passages worn 

 by solution along the line of intersection of two joints or joint and 

 bedding planes. Some tabular forms assume the dimensions of veins. More 

 irregular shapes are often referable to the formation of chambers by 

 solution where a close congeries of cracks has comminuted the chalk and 

 rendered solution more rapid. The cylindrical flints referred to are 

 those which are popularly known as '^fossil bones." ^ Like other flints, 

 they are occasionally hollow, and this suggests that they are formed by 

 the filling of a previously formed cavity rather than the usual radial 

 o-rowth of concretions bv crvstallization which orioinates round a central 



* A view held by Buckland and Lyell. See Lyell's "Elements of Geology." page .321 

 et seq., for a comlDlnatlon of depositional and concretionary hypotheses, and also as to 

 source of silica and origin of potstones. 



° As to forms of flints and relation to fissures, see article by J. Smith Flett, entitled 

 "Flint,'" Encyclopedia Britannica, eleventh edition. 



