444 Mr. W. J. Sollas on the Flint 



doubt ; but as to the means by which it has been effected we 

 have still much to learn. Alexis A. Julien, in a paper of the 

 highest importance on the geologic action of the humic acids, 

 suggests * that albuminoid or glairy matters and acids akin 

 to the azohumic of Thenard, produced during the submarine 

 decomposition of organic matter, may have been the agents 

 which accomplished the solution. This may very possibly 

 have been the case, though possibly the water at the sea- 

 bottom may, even without the assistance of these substances, 

 have been a sufficiently powerful solvent ; and this appears 

 the more likely when we consider the considerable pressure 

 under which such water exists, even at depths no greater than 

 that under which the Trimmingham spicules were dissolved, 

 the depth of water which we have indicated for them (100 to 

 400 fathoms) giving a pressure of from 20 to 80 atmospheres. 

 An observation of Carter's tends to bear this opinion out ; for 

 some spicules which he examined, from depths not much 

 greater than those under which ours were formed, were found 

 to exhibit the usual signs of incipient solution, such as pitting 

 of the surface and enlargement of the axial canals. Yet these 

 spicules came from an area swept by a marine current, where 

 organic matter was presumably not plentiful. The bottom- 

 water of the sea is remarkably free from organic matter; and 

 in this case we probably have to do with solution under pres- 

 sure. Again, the rapid whitening of the black surfaces of 

 freshly broken flints when exposed to the weather, as in the 

 case of the flint walls in Cambridgeshire, seems to show that 

 even pure rain-water is of itself capable, without any aid 

 from pressure, of dissolving a form of silica much less soluble 

 than that of sponge-spicules. It is true that the presence of 

 a certain qnantity of lime in the flints may have rendered 

 them more liable to the action of slightly carbonated water 

 such as rain-water, though, on the other hand, the exceedingly 

 small proportion of lime present, as shown by analyses, may 

 make us hesitate in attributing any great influence to it. 



(iv) The redejjosition of Silica. — After the silica of sili- 

 ceous skeletons has passed into solution, it is again extricated 

 in the solid state ; and, since both the deposition and solution 

 take place in the same deposit, a seeming difficulty presents 

 itself, since one would have thought that the conditions which 

 led to the one would have been incompatible with the occur- 

 rence of the other. An explanation is to be found in the fact 

 that the one process is not merely a reversal of the other, and 

 in the possibility that both did not take place at the same 



* Proc. Am. Assoc. Adv. Science, xxyiii. p. 396, Saratoga Meeting, 

 1879. 



