Flints of the Upper or White Chalk, 181 



sions may not have been strictly accurate : — " It is probable 

 that the saline and mineral substances present in sea-water 

 exercise a much more marked effect on the formation of the 

 organic deposits of the deeper zones of the ocean than has been 

 admitted under the ' antibiotic ' view so often referred to. 

 From the nature of the difficulties by which the inquiry is 

 surrounded, not only is the chief portion of our knowledge 

 regarding the deep-sea bed rather of a theoretical than a prac- 

 tical kind, but unfortunately it must long continue to be so. 

 It is therefore doubly expedient to test this knowledge by 

 the light of every fact that science or accident may throw in 



our way If we examine the siliceous concretions, our 



perplexities increase rather than diminish ; for whilst remains 

 of siliceous-shelled organisms are to be met with in them, it is 

 very remarkable that they do not belong to the family of 

 siliceous-shelled Rhizopods that next to the Foraminifera are 

 most largely represented at the bed of the ocean, namely the 

 Polycystina; and there is no authenticated example up to the 

 present period of a Polycystine shell having been detected in a 

 jlint. From the nature of the hydrosilicates^ we could hardly 

 expect to find i\ie, forms of siliceous organisms preserved ; hence 

 it is possible that the mineral atoms of the Polycystina have 

 become merged as it were into the substance of the masses. 

 But since we constantly detect siliceous spicviles of sponges, 

 which have not yielded to disintegration though similarly 

 formed, it is difficult to reconcile the apparent anomaly. If 

 we regard the concretions as principally made up of sponge- 

 epicules, the case is but little altered ; for the pseudomorphs of 

 the calcareous shells of the Foraminifera are plentiful in their 

 substance, and indicate that the conditions under which they 

 were formed and silicified were such as might have been 

 shared by the testaceous Rhizopods generally " [op. cit. 

 pp. 120, 121). "Again, in those marine deposits in which 

 the Diatomacefe are sufficiently abundant and well marked to 

 indicate that they had lived in the immediate locality, it may 

 be taken for granted either that the water was shallow or that 

 the deposit was formed along a coast-line, since no Diatomacece 

 live at greater depths than from 400 to 500 fathoms. In the 

 deep-sea beds where Diatomaceae occur, the characters of the 

 species, their variety, and their limited numbers, at once show 

 they had been drifted from distant shallows, or were free 

 floating surface forms which had subsided to the bottom after 

 death. Whilst as yet we have no positive proof that the 

 Polycystina live at extreme depths, it is a very significant 

 circumstance that the large assemblages of these organisms 

 hitherto met with in such a recent state as to indicate vitalitv 



