88 DR. WALLICH ON THE PHYSICAL 



have been singularly insufficient, however valuable they may be (and 

 valuable they unquestionably are) to the naturalist and biologist*. 



It is therefore quite possible that on analyses being made of mud 

 procured from oceanic areas free from any source of exceptional 

 mineral admixture, the percentage (30 per cent, or thereabouts) of 

 soluble and insoluble siliceous materials which has been supposed 

 to apply to the Atlantic mud generally may much more closely ap- 

 proximate to the small residuum which is known now to exist in 

 the Chalk. In connexion with this subject I cannot refrain from 

 quoting an admirable passage in Professor Prestwich's Address, 

 to which I have already been so deeply indebted. It is as 

 follows : — " In one point of view, the geologist has the advantage 

 over the naturalist. The latter examines the coasts, and dredges 

 in the ocean, but he can only skim the surface, whereas the former 



has the old sea-beds opened out to him What may be under 



the surface of the Atlantic mud we know not. Is there a succession 

 of strata extending down to the equivalents in time of our chalk 

 strata ? or would the equivalent of the latter prove to be merely 

 one part of a series, the other end of which would convey us back 

 to the Oolitic, Jurassic, Triassic, or even Carboniferous times ? . . . . 

 The present explorations, fuU of interest and valuable as they are, 

 are insignificant compared with the vast area of the ocean " (loc. cit. 

 pp. 49, 50). 



It now only remains for me to complete the present preliminary 

 sketch of the agencies concerned in the formation of the flints from 

 the materials present wherever the calcareous mud of the Atlantic 

 is to be found, by stating that the Stratification of the Plints is due 

 to the fact, already touched upon in a previous page, that nearly the 

 whole of the silex derived from the Sponges on the one hand, and 

 the continual subsidence of minute dead siliceous organisms on the 

 other, is retained in the general protoplasmic layer which I have 

 shown maintains its position on the immediate surface of the cal- 

 careous deposit, and gradually dissolves the silex. This layer, in 

 virtue of its inferior specific gravity, rises with every increase in the 

 thickness of the deposit, until, at last, the supersaturation of the 

 protoplasmic masses with silex takes place, and the first step to- 

 wards the consolidation into fiint is accomplished — the continuity of 

 sponge-life, and of the various other forms which tenant the calca- 

 reous areas, being secured through the oozy spaces which separate 

 the sponge-beds, and thus admit of both adult and larval forms 

 having free access to the overlying stratum of water. 



That the predisposition of silica, itself in reality a colloid, to 

 form colloidal combinations with albuminous and other materials 



* In 1863, at a Meeting of the Royal Geographical Society, I exhibited 

 various forms of deep-sea apparatus, and amongst these a Pelimeter, specially 

 designed by me for penetrating the deep-sea deposits to a considerable distance 

 and bringing up a core about 2 inches in diameter and from 10 to 15 inches in 

 length. The instrument was also adapted for giving distinct indications of 

 rocky bottom. This Pelimeter was highly spoken of by Sir Eoderick Murchison 

 on the occasion referred to. (See ' Proc. Eoy. Geograph. Soc' vol. vii. No. 2, 

 1863.) 



