Prof. Grenville Cole — Rhythmic Deposition of Flint. 65 



a very casual way of dealing with the difficulties of the problem. 

 Gr. C. Wallich ' in 1879 anticipated Mr. Bulman's suggestion of the 

 periodic destruction of sponge life ; but his hypothesis allows of 

 a prolonged time-interval between the formation of successive layers. 

 His paper is not systematic and does not provide easy reading ; but 

 the following argument may be picked out from it and pieced 

 together. Ordinary Globigerina-ooze contains but little silica, like 

 the beds of chalk between the flint-zones ; oceanic dredgings, how- 

 ever, show larger amounts of silica, because they represent only the 

 superficial layer of the deep-sea ooze. This layer contains a large 

 amount of protoplasm floated up from the foraminifera, including 

 Globigerinse, that multiply on the sea-floor. Sponges flourish in this 

 layer and add the silica of their spicules to it ; but a great addition of 

 silica is made by the raining down of radiolarian skeletons from 

 surface-waters. The radiolaria live only at the surface, and their 

 remains are caught in the protoplasmic layer, which thus becomes 

 more and more highly charged with silica. The protoplasmic masses 3 

 " will become, if not supersaturated with silica, at all events so highly 

 charged with it in a now colloid state more and more closely 

 approaching coagulation, as eventually to asphyxiate — so to speak — 

 the very organisms which have produced them". 



Owing, no doubt, to the paucity of thin sections of flint at the date 

 at which he wrote, Wallich shows no appreciation of the fact that the 

 silica of flints replaces calcareous organisms and that a flint-zone may 

 prove to be a pseudomorph of a consolidating bed and not a record of 

 a differentiated layer on its surface. In his reply to the discussion 

 (p. 92), he somewhat vitiates his previous argument by introducing 

 a new point, the attraction by sponges of colloidal silica "which 

 existed at the surface of the mud". The paper, however, is clearly 

 an attempt to meet the question of rhythmic deposition. In recent 

 years, however, zonal crystallization has become realized through 

 a number of attractive experiments, notably those of It. E. Liesegang, 

 and a new field of thought has opened in regard to our familiar flints. 



Mr. Bulrnan, 3 after laying stress upon the annual repetition of 

 sponge-deposits, quotes some correspondence with myself in regard to 

 Liesegang's work, and urges that the prevalence of sponges along 

 particular horizons would in any case be necessary to account for the 

 localization of the silica. He concludes, " I make these remarks with 

 the reservation that there may he in Liesegang's work, and in Professor 

 Cole's suggestion, details of which I am not aware, and which might 

 render them a more complete explanation of the lines of flint than 

 they appear to be." 



Since Liesegang's Geologkche Diffusionen (Dresden, 1913) is not 

 widely accessible in .our Islands, it may be well to mention how the 

 question of flint-horizons is introduced in that highly suggestive and 

 stimulating work. 



One of the fundamental propositions governing the deposition of 



1 "A Contribution to the Physical History of the Cretaceous Flints": 

 Quart. Journ. Geol. Soc. London, vol. xxxvi, p. 68 : 1880. 



2 Op. cit., p. 82. 



'' Op. cit., Science Progress, No. 41, p. 156. 



DECADE VI.— VOL. IV. — NO. II. 5 



