242 Prof. Ansted on the Zoological condition of Chalk Flints, ^c. 



am myself inclined to take, and which I am now about to advo- 

 cate, is borne out entirely by positive observation. I think, how- 

 ever, I shall be able to offer a reasonable and probable account, 

 and one more consonant with the results of minute investigation 

 than any of those I have quoted. 



Of the lithological and geological character of the chalk and 

 the layers of flint imbedded in it I need say vei-y little. Chalk 

 itself is a nearly pure carbonate of lime, having a small quantity 

 of iron as well as silica distributed through it, the particles of the 

 former mineral being collected into nodules irregularly placed, 

 and usually crystallized with sulphur in the form of pyrites*. 

 Careful microscopical investigation has distinctly proved that a 

 large proportion of the solid chalk is of animal origin, and the 

 substance most nearly resembling it in a recent state is a white 

 calcareous mud formed in coral lagoons by the decomposition of 

 recent soft corallines. The flint in chalk consists of about 98 per 

 cent, of pure silica, and is usually of dark colour internally, but 

 each nodule or separate portion of a layer is surrounded with a 

 thin coat of a white colour and a coarser or more saccharoid tex- 

 ture, evidently owing to the presence of a certain proportion of 

 chalk. The flints occur either in detached nodules or tabular 

 masses ; the latter forming layers of various thickness alternating 

 with a much greater though also variable thickness of chalk, and 

 the former more rare and still having a tendency to stratified ar- 

 rangement. Flints are also sometimes found filling up veins and 

 traversing the chalk at various angles. Almost the w^hole how- 

 ever of the siliceous matter of the upper chalk is distributed in 

 tabular masses, and it is chiefly therefore to those that I shall 

 refer, although I may mention here, that there is no apparent dif- 

 ference in structure between the tabular flints and those which 

 occur detached or filling up veins. 



In order to discover the conditions under which siliceous matter 

 may have formed in the chalk, and the cause of its appearing in 

 layers regularly stratified and distinctly separate, it is clear that 

 we must investigate minutely not only the chemical condition of 

 the mineral, which is a very insufficient guide, but also its mecha- 

 nical condition ; and this must be done in two ways, examining 

 . first its structure by the aid of a powerful microscope, and then 

 observing its external relations both with the chalk itself and also 

 with the organic bodies imbedded in it. We must also pay some 

 attention to the nature of those silicified fossils of whose organic 

 origin there can be no doubt. 



* A small proportion of argillaceous matter occurs in the lower chalk, 

 but this proportion is smaller as we examine specimens higher up in the 

 formation, until at length in the white chalk with flints it can no longer be 

 traced. 



