Dec. 21, 1888.] 



SCIENTIFIC NEWS. 



6 37 



many fossils which they found in flints. These fossils 

 were sea urchins, very much like in structure 

 some of the sea urchins living at the present 

 day — sea-eggs some people called them — but really 

 different in species. Now .in these cases the original 

 sea urchin was completely filled with flint, and it was so 

 completely filled that the holes through which the arms 

 of the animal passed, stood out as little needles on the 

 surface, but the shell itself was not represented. The 

 shell itself was represented by a hollow space ; the 

 outside of the shell had left its markings on the flint. 

 There was what was called an external cast, and where 

 the original shell was there was an empty space. The 

 reason of that was that the shells of these animals were 

 composed of this comparatively stable substance, calcite, 

 but all round was the unstable chalk-mud. Now this 

 chalk mud most easily passed into solution, and was 

 therefore silicified. The calcite of the shell being more 

 stable resisted this process and therefore remained as 

 carbonate of lime. Occasionally they might find flints 

 with shell remaining as carbonate of lime, but in other 

 cases it would be found that water had percolated 

 through the mass, after the silicification of the other 

 part had been completed and had dissolved out of the 

 calcite, leaving a hollow space. There was in the case 

 on the table one of the most curious burrowing sponges. 

 This sponge had burrowed into the shell, and the result 

 had been that the hollows had been filled by that chalk 

 mud, and wherever those hollows existed, silicification had 

 taken place ; but in the shell around the cell, silicification 

 had been resisted, and afterwards the shell had been dis- 

 solved away, and thus they had got the curious appearance 

 that was presented. There was no end to the number of 

 very interesting features which they might find exhibited 

 in shells fossilised in flint, and he hoped that the facts they 

 had been considering would enable them to interpret 

 them. 



He must confess that although it was perfectly clear 

 that flints were silicified mud, and that silicification 

 had taken place by the series of processes he had 

 described — he must confess that there was a problem 

 still unsolved — why did silica concentrate itself along 

 certain bands, and form certain nodules ? That problem 

 was still an unsolved problem. There were several old 

 experiments which were tried many years ago, but which 

 were unfortunately not confirmed. They must not 

 rush too hastily to the conclusion that the author of these 

 experiments made a mistake. He took a material like chalk 

 mud through which he distributed a small quantity of dis- 

 solved silica — matter containing silica in solution. He 

 then put a number of minute bodies consisting of silica, 

 bodies like sponge spicules, in the hope that these would 

 serve as centres of aggregation — that where these siliceous 

 bodies were, the silica would tend to collect in 

 masses analogous to chalk flint. Probably, as it 

 seemed to be an experiment which was not yet 

 fully confirmed, they must admit that there was 

 still a problem remaining unsolved with regard to these 

 shell flints. 



Now, he must call their attention to the fact that 

 many flints did not exhibit the characters of silicified 

 chalk mud, and in order to explain the reason of this 

 he must recall their attention to the doctrine of allo- 

 morphism — a doctrine that substances having the same 

 composition might have their molecules so arranged 

 as to form a substance with totally different properties. 

 A most remarkable example of this was frequently 



pointed out to them in the case of silica. Silica existed 

 in two forms. Colloidal silica, which was unstable and 

 easily passed into solution, was unstable because it 

 was constantly tending to pass over into the other 

 form, crystalline silica ; and crystalline silica, or quartz, 

 which was stable — almost insoluble. There was a 

 constant tendency for this unstable colloidal silica to pass 

 into the stable form known as quartz. Perhaps he 

 could illustrate these paramorphic changes by an 

 experiment with a substance he had with him, 

 which like silica had a great tendency to pass into 

 another state ; this substance was mercuric iodide. If 

 they examined a little of that material under a micro- 

 scope, they would find it consisted of red crystals of very 

 definite shape, but he could make it assume a different 

 form of crystal which had a bright yellow colour, and 

 make it pass into the allomorphic form, though the com- 

 position of the material did not change. One of the 

 ways in which it might be made to assume the allomor- 

 phic form was by heating it. They saw by the simple 

 action of heat the red substance had passed into its allo- 

 morphic yellow form. Now the red substance was a stable 

 form, and the yellow substance was the unstable form. 

 The stable answered to crystalline quartz, and the un- 

 stable to colloidal silica. The yellow was a particu- 

 larly unstable substance, and mechanical pressure would 

 cause it to change. He would take a hard substance, 

 and they would see that wherever the hard substance 

 had touched those yellow crystals, they had instantly 

 broken up and arranged themselves in the forms 

 of new crystals which had a bright red colour. 

 Although there were not many cases which by 

 a change of colour this paramorphic change was 

 manifest, such paramorphic changes were continually 

 going on. If he left the paper with which he 

 had been experimenting for three or four days, he would 

 find the whole of the yellow material had gone over to 

 the red material— that paramorphic change had taken 

 place spontaneously, so unstable was the yellow material 

 that he had produced. Now in the same way the 

 colloidal silica which was deposited in sponge spicules 

 was constantly tending to pass over into crystalline silica 

 or quartz. When they examined any of the rocks which 

 were made up of sponge spicules, with a microscope, 

 they constantly found the sponge spicules re-arranging 

 themselves in crystalline form. The first change which 

 took place when masses of chalk mud were silicified 

 was that colloidal silica gradually passed particle by 

 particle into minute crystals, forming bodies known as 

 microlites. These microlites built up structures which 

 were of great beauty, and which were at once recognised 

 by the fact that they polarised light when examined 

 between cross angles ; instead of having a dark body, 

 they polarised in wonderfully brilliant colours. The 

 white flint which was found in the centre of chalk 

 flints, and on the outside of chalk flints — this white flint 

 was formed by the silicification of the white chalk 

 mud, and, like the white chalk mud, it was white, and 

 it retained its colour, although a greater or smaller pro- 

 portion of the siliceous replacement might pass into a crys- 

 tallised form by this paramorphic change. But when this 

 white flint has this colloidal silica added to it, the 

 change produced was exactly like that change which was 

 produced when a piece of blotting paper was put into 

 water, or still better into oil. In that case opaque 

 blotting paper became translucent, and when they dried 

 it again it became opaque. The introduction of this 



