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SCIENTIFIC NEWS. 



[Dec. 21, li 



What they had seen of the microscopical structure and 

 character of flint would convince them that though flints 

 consisted entirely of silica, they were nevertheless not 

 deposits made up of radiolarians and sponges, or of 

 diatoms. Now let them consider if it was possible that 

 the materials which had been collected to form these 

 organisms could be by any process collected so as to 

 form these nodules and bands which were found in the 

 chalk, and which were called flints. He had also 

 called their attention to the way in which those nodules 

 and bands occurred in the chalk ; he had also 

 pointed out to them a series of chemical analyses which 

 showed that when there existed chalk without any 

 flints they found silica distributed through the 

 mass, but that chalk with flints did not contain silica 

 distributed through it, and the suspicion was at once 

 aroused that by some process or other the 

 silica which formed the organisms deposited with cal- 

 careous materials which formed the chalk mud ; that 

 this silica had been separated by some means and col- 

 lected into these nodules. Now let them see if there 

 were any grounds for believing that such takes place. 

 He had already pointed out how readily colloidal silica 

 passed into a state of solution, and he had also brought 

 before them evidence that this dissolved silica was every- 

 where present in different kinds of rocks. They would 

 recollect that he showed them how old limestone rocks, 

 when dissolved, were found to contain little crystals of 

 quartz, which must have been formed from silica dis- 

 tributed in a state of solution through those rocks, or, 

 rather, in solution in water, that percolated through 

 those rocks. In the same way he had shown them how 

 sand-grains, which were broken fragments of quartz 

 crystals, were continually being formed into perfect 

 quartz-crystals by the deposition upon them of silica, 

 which had been also in a state of solution. He had 

 shown them, too, that, when flints got broken, they 

 became cemented together again. If they tested that 

 with acid they would find the cement was siliceous 

 cement. It was silica, which had been deposited to 

 repair the flint ; so that they readily saw that the col- 

 loidal silica forming these siliceous organisms in a mud 

 such as that which gave rise to the chalk might easily 

 pass into solution ; and he showed them a case where, 

 in globigerina-ooze, the silica forming these organisms 

 had clearly passed into a state of solution and been de- 

 posited in a fresh form. 



Now, he must recall to their mind the facts to which 

 he pointed with regard to silicification. They would 

 recall the case of a plant he brought before them — there 

 was no mistake about it being a plant originally. He put 

 before them a section showing all the cells and vessels 

 having their different forms, and the whole was perfectly 

 silicified. Particle by particle the carbon and the other 

 elements present in the wood had been dissolved away, 

 and each molecule of the complex organic compound had 

 been removed, and the molecules of silica had been put 

 in its place, so that the whole structure had been 

 built up just as a red brick building would be restored 

 if one knocked out a red brick and put in a white, 

 and did this with every brick in succession until the 

 whole of the original red brick building had been re- 

 moved and the white brick, having exactly the same form 

 and characteristics of the red brick building, pnt in its 

 place. This was what went on on a finer scale in nature 

 when a substance was undergoing silicification. The 

 original structure, particle by particle, was removed, and a 



fresh particle of silica was put in its place until in the end 

 there was scarcely a vestige of the old substance. 

 There was nothing but silica, but this silica presented 

 all the forms that were found in the original mass ot 

 wood, or whatever it was that was silicified. He must 

 ask them to bear in mind how completely this process 

 of silicification took place. 



The result of the study of all those microscopic 

 sections to which he called their attention the other 

 day, was to show that flint was silicified chalk mud. 

 Chalk mud consisted essentially of the substance 

 calcic carbonate, or carbonate of lime, but this calcic car- 

 bonate of the chalk mud can be silicified particle by par- 

 ticle. The carbonate of lime might be dissolved and 

 silica put in its place, and the whole mass become a mass 

 of silica. This chalk mud was sometimes found perfectly 

 silicified and formed into flints. Every particle of each 

 of these rounded shells is dissolved and particles of 

 silica put into its place, so that but for the fact that it does 

 not dissolve in acids they might at first sight think they 

 were dealing with actual chalk, but they were dealing 

 not with actual chalk but with silicified chalk — a piece of 

 chalk which had undergone precisely the same change as 

 the silicified wood had undergone. Now it was very 

 true that all flints that were examined did not look like 

 chalk mud. There were many flints in which evidently 

 a very great change had taken place, and the way in 

 which that change had taken place he must proceed to 

 show them, but from these changed flints to those 

 which were manifestly nothing but chalk mud, 

 there was every possible graduation. They found 

 some flints which if put side by side with a piece 

 of chalk, and in thin section, appeared so much alike that 

 they would say there was no difference between them, 

 yet the one was chalk and the other was flint. There 

 could be no doubt that silicification had taken place 

 because these were all forms of shells which were 

 contained in carbonate of lime. There were no siliceous 

 shells like these. It was clear that these shells had been 

 originally carbonate of lime, and had been converted into 

 silica by this wonderful process of replacement and 

 silicification. Now, he must ask them to accept the fact, 

 which was the result of an enormous number of obser- 

 vations of different specimens, that from the flint which 

 was most obviously and clearly nothing but silicified 

 chalk mud, they had every possible gradation in the 

 flints that showed none of this structure. He must just 

 call their attention for a moment to some of the curious 

 phenomena seen during the silicification of the chalk mud. 



He had already pointed out to them how curious were 

 the results that arose from heteromorphism — the 

 same forms which the same substance silica could 

 assume. We have two totally different substances ol 

 different properties of the most startling character, and 

 both consisted of silica. There were other substances 

 besides those two which were especially important — the 

 crystalline silica known as quartz, and colloidal silica. 

 Now, just as silica existed in two different stftes with 

 different properties, so calcic carbonate or carbonate 

 of lime, which formed chalk mud, existed in two forms. 

 There was the crystalline form, known as the mineral 

 calcite, and there were the less stable forms known as 

 aragonite. He wanted them to recognise that there 

 was a form which was stable, and which was not 

 liable to change, and which was not easily passed into 

 solution, and there was another that easily changed and 

 passed into solution. They saw the effect of that in 



