442 Mr. W. J. Sollas on the Flint 



believe, have such of the Trimmingham flints as still retain 

 a definite form. At the same time they have evidently 

 received an addition to their proper spicular complement from 

 sponges of other kinds; and we have still to consider whether 

 these additional spicules were collected by current-action. 

 That currents may have had some influence is clear enough. 

 The flints were not formed at any abyssal depth ; as we have 

 seen, the associated spicules indicate limits of 100 to 400 

 fathoms ; and Carter states that even at greater depths consi- 

 derable drifting is produced by currents. Thus he says * : — 

 " The dredgings of H.M.S. ' Porcupine ' indicate, through the 

 specimens now with me, that about 100 miles north of the 

 Butt of Lewis, in 632 fathoms (station no. 57), there must be 

 a bed of sponge-spicules of many kinds, portions of which are 

 rounded by the currents into pebble-like forms, which may 

 one day become the nuclei of flints." 



The observation of Sorby that some specimens of chalk 

 seem to show signs of a gentle washing- action, and the 

 occurrence of a few small grains of quartz-sand in the Trim- 

 mingham chalk, are both evidences favouring the idea of 

 current-action. Still I do not think that drifting has occurred 

 to any great extent. The spicules are not sorted out and col- 

 lected into a purely siliceous layer; but such of them as remain 

 are intimately mixed with granules, coccoliths, and Foramini- 

 fera, which do not differ from ordinary chalk material, except 

 in being partly siliceous; in other words, the separation which 

 drifting might be called in to accomplish has not taken place. 

 In like manner the different spicules themselves are con- 

 fusedly mixed together, large and small alike, with no ten- 

 dency for the small to occur in one place and the large in 

 another. The once existing flesh-spicules, it is true, are absent 

 — not because they have been washed away, however, but 

 dissolved; for they are invariably absent in fossil sponges 

 and stratified deposits. Neither Zittel nor I have seen a 

 trace of them ; and my observations on the comparative readi- 

 ness with which they undergo solution in caustic potash 

 serve to explain their absence. If drifting has taken place, it 

 must have been to a very slight extent, sufficient to help in 

 mixing the different sponge-spicules together, but not to sort 

 them out into any distinct layer. 



Our belief is that the area over which the Trimmingham 

 spicules are now found was once a sponge-bed, where nume- 

 rous sponges flourished, generation after generation, in a 

 luxuriant meadow-like growth ; many of them led a parasitic 



* Ann. & Mag. Nat. Hist. ser. 4, vol. xvi. p. 40 (1875). 



