HARDWICKE'S SC lENCE- GO S S IP. 



2<1.5 



spicules would possibly exert an attractive in- 

 fluence upon the atoms of silica in solution, in a 

 similar way tliat a crystal of saltpetre would be the 

 startiug-poiut for further crystallization in a solution 

 of that salt. 



Another and still more effectual cause of the 

 elimination of silica would be the decomposition of 

 the sarcode and keratode material ; as this goes on 

 certain gases are produced, and the silex preci- 

 pitated from the solution. 



The discoveries of Wallich, Carpenter, and others, 

 of protoplasm or sarcode existing at the bottom of 

 the ocean, and to which I have previously alluded, 

 appear to alford a prcbable explanation of the 

 cause of solution to the flinty layers found in some 

 of the chalk strata. The opponents to the spongeous 

 origin of flints brought forward the existence of these 

 layers as a proof that sponges were not the nuclei of 

 fiiuts, and until the existence of this free sarcode 

 was detected, the occurrence of flints in some 

 localities in the form of nodules, and, in others as 

 layers, was difficult to account for; but when it was 

 found that sarcode existed in masses covering a 

 considerable area, a clue to the formation of the 

 flint layers became apparent. Carbonic acid gas 

 and hydrogen were liberated when decomposition 

 set in, and silex replaced the sarcode. That flints 

 are now forming, is, I think, as certain as the 

 formation of new beds of chalk ; and it is an ascer- 

 tained fact that valleys in the bed of the ocean 

 are, as in days of yore, being gradually filled with 

 calcareous matter intermingled with remains of 

 sponges, or permeated with protoplasm : these, as 

 decomposition slowly takes place, are separating the 

 silica from the surrounding waters. 



Fig. 172. Polypothecia x 150 diameters, green sand, 

 Warminster, Wilts. 



It will, I think, be allowed, that flint nodules 

 could not have been found in chalk unless a nucleus 

 had existed ; the silica contained in the water was 

 chemically combined with it, the chalk only me- 

 chanically, and if any silica was parted with, it 



would only act as silicious cement hardening the 

 atoms of chalk. 



The probability that flints are still in the process 

 of formation is confirmed by the frequent discovery 

 of silicious casts of foraminifera ; those usually found 

 are composed 'of silex, with traces of iron, giving 

 them an olive-green colour, precisely like those 

 found in the green sand. 



In a dredging made at Porto Seguro by Capt. 

 Perry, of Ifiverpool, I found many shells of forami- 

 nifera, which, when acted upon by acid, showed the 

 interior filled with a silicious cast of the internal 

 chambers, and in some specimens even the pseudo- 

 podal apertures had also been filled with silex ; 

 fragments of other silicified organisms were also 

 of frequent occurrence. The casts found in this 

 dredging differed from those usually found by the 

 absence of any trace of iron, and appeared to be 

 silica in a similar condition to the ordinary chalk 

 flint. 



Fig. 173. Polypothecia x 150 diameters, g-reei sand, 

 Carrow, Norwich. 



In the green sand large silicious nodules, known 

 as Polypothecia, are of frequent occurrence, and 

 when thin sections are examined their spongeous 

 origin is distinctly seen. These nodules were, how- 

 ever, formed under somewhat different conditions 

 to the ordinary chalk flint ; the silica is distinctly 

 crystalline and doubly refractive, and polarizes like 

 quartz or agate ; the sponges were also probably 

 different from those belonging to the chalk. A 

 careful microscopic examination of very many sec- 

 tions did not reveal the presence of any form of 

 spiculum; they.were most likely aUiedto the recent 

 keratode sponges ; in fact, a thin slice of ordinary 

 domestic sponge greatly resembles a section of its 

 silicified predecessor. The reticulations are not 

 solid, but tubular, and I have been able, in many 

 cases, to fill them with colouring matter. 



The above figures are from camera lucida 

 drawings of sections of Polypothecia ; —fig. 172,Trom 

 the green sand, Warminster ; fig. 173 from a frag- 



