116 



EECREATIVE SCIENCE. 



able difficulty in understanding how it could 

 be precipitated in the manner we now behold. 

 One ingenious and highly probable theory 

 has been proposed, which is briefly this : — 

 The common tuberous flints and the hori- 

 zontal tabular flints, together with those 

 forming oblique or vertical veins, were all 

 produced by the same agency. We know, 

 from the frequent remains of sponges now 

 distributed throughout the chalk, that these 

 bodies were abundant in the sea that held 

 the chalk in solution — so abundant that their 

 remains are almost everywhere present in cer- 

 tain beds and localities. Wherever a sponge 

 settled upon shells or other organic bodies in 

 the chalk ocean, it gradually enveloped it 

 and grew round it more or less completely. 

 It coated over any such mass upon which its 

 gemmule might chance to settle, in a manner 

 precisely analogous to the habits of the fresh- 

 water sponge of our rivers, and to many other 

 parasitical species which are inhabitants of 

 the sea at the present period. As the sub- 

 atance thus built upon was probably, to a 

 small extent, immersed in silt or mud, we 

 rarely find more than half or two-thirds of the 

 surface enveloped, and it is from this circum- 

 stance that we detect in chalk so many fossils 

 which are, more or less, imbedded in flint. 

 It is presumed that any flint found in the 

 chalk formation was a mass of casing-sponge, 

 which gradually accumulated round some 

 organic nucleus, and either left the original 

 body unchanged in shape, as we often find 

 it, or in part transmuted the same into its 

 own substance, leaving only tokens of the 

 organism which may be detected by micro- 

 scopical examination. As many of the or- 

 ganisms would be broken up before the 

 gemmule of the casing-sponge fastened upon 

 them, only fragments remain in the flint, and 

 sometimes only impressions upon its outside. 

 We select, for illustration, specimens 

 of chalk flints fiom our own cabinet and 

 such as may be ordinarily met with by any 

 observant pedestrian, either in clialk districts 



or in gravel districts, such as are common in 

 the vicinity of London, where the' gravel is 

 composed of flints derived from the chalk : — 



1. An echinus of solid flint, from the chalk 

 cliffs, Margate. In this specimen the mark- 

 ings are particularly distinct, and it is evident 

 that the liquid flint has filled up the original 

 echinus, which has decayed away, leaving 

 this interior cast of it. 



2. A similar echinus, from the London 

 gravel, Regent's Park. Althougli this echi- 

 nus has been rolled about in its after course 

 from leaving the chalk, yet it is as distinct as 

 the former specimen Mhich came at once from 

 the flint in situ. 



3. A beautiful little echinite {Diadema 

 depressa), from a chalk flint in the road on 

 Haverstock HUl. Flint has entirely filled 

 and surrounded it. 



4. A bivalve shell in flint, from Margate. 

 In this case the half-enveloping flint was 

 large, and the shell could not be made port- 

 able without fracturing the flint. The shell 

 projected some distance from the mass. 



5. A sponge enveloped in a flint casing, 

 from the gravel of London. The net-like work 

 of the sponge has been fully filled with silex. 



6. A ventriculite (from ventriculus, a ven- 

 tricle or sac) completely encased in flint, or 

 perhaps a mere flint mould of the ventriculite, 

 from the London gravel. This is a prevalent 

 form in the gravel derived from the chalk. 

 Probably all such flints have been moulded 

 in the cup-like cavities of zoophytes. The 

 top displays the impressions of the reticu- 

 lated outer surface of the original, the form of 

 which is conceived, from numerous specimens, 

 to have been a hollow inverted cone terminat- 

 ing in a point at the base, which was attached 

 by fibrous rootlets to other bodies. The 

 outer integument was disposed in meshes like 

 net-work, and the inner surface was studded 

 with irregular openings, which aj)parently 

 were the orifices of tubular cells. When the 

 flint filling up the cavity of a ventriculite can 

 be extracted, it is a solid cone. 



