Nodules of the Trimmingham Chalk. 459 



4. Lastly we have to consider the irregular nodules of 

 flint. These, by their fantastic flowing outlines, are respon- 

 sible for much of the theorizing which can only regard flint 

 as a silicification of organic matter. Thus Dr. Wallich 

 repeatedly lays stress on " the unique amcebiforra nodulation of 

 the flints," — though one may remark that one of the charac- 

 teristic features of an amcebiform outline is that it seldom 

 remains the same two minutes together ; and this cannot be said 

 of flints, although, as Dr. Wallich speaks in another place of the 

 flints showing " signs of the specific contractility of colloid 

 silica," one might infer that he does not regard this character 

 as absent. A flint moving by means of its pseudopoclia would 

 indeed be an interesting object; but perhaps the distinguished 

 writer merely alludes to the excessive shrinking which colloid 

 silica undergoes in passing from the pectous to the solid state ; 

 and certainly to one who has experimented with colloid silica, 

 the wonder on Dr. Wallich's hypothesis would be, not that 

 the flints show signs of shrinkage, but that they do not present 

 them more markedly. The time for conclusions based on 

 superficial resemblance is now gone by ; we no longer regard 

 " dendrites " as fossils on account of their moss-like form, nor 

 profess to be " able to tell an honest man by the smell." 



The direct action of organic matter seems to be excluded by 

 the great lapse of time which would be required for the solu- 

 tion of sponge-spicules, and during which the organic matter 

 would decompose and wholly disappear. 



Many concretions exhibit an irregular form besides flints 

 (the cornstones of the Old Red Sandstone for instance) ; only 

 the irregularity is carried further in the nodular flints than in 

 most cases. 



The form of the nodules simply indicates the irregular dis- 

 tribution of siliceous solutions about an irregular bed of 

 sponge-spicules, at the time they replaced the surrounding 

 chalk and deposited silica in its interstices. 



This paper has much exceeded the length I proposed 

 on commencing it ; and I will only remark in conclusion 

 that at length, out of much that is uncertain, some few fixed 

 points in the history of flints begin to appear. That the 

 silica composing them has been derived from sponge-spicules 

 is no longer a mere assertion, but a well-ascertained fact; that 

 it has been deposited in the first place as a pseudomorph after 

 carbonate of lime is also clear, and no less so that subse- 

 quently a simple deposition of silica converted the siliceous 

 chalk into flint. Various causes have determined the external 

 forms of flint, chiefly the distribution of the spicules which 

 have furnished it, but partly the existence of open fissures 



32* 



