614 



SCIENTIFIC NEWS. 



[Dec. 14, if 



separated and deposited in the form of crystalline quartz. 

 [Another example of the way in which silica was de- 

 posited in rocks, owing to its being continually in a state of 

 solution, was shown by the lantern.] There were a num- 

 ber of sand grains which were remarkably opaque, be- 

 cause they were covered with iron rust or iron 

 oxide; but if they were put into«.cid the iron rust was 

 dissolved, and they got a crystalline form. The sand 

 grains were little fragments of quartz crystals, and when 

 silica in a state of solution came into contact with them 

 the broken crystals were reformed, and thus they had 

 that wonderful deposition of silica upon the outside of 

 the sand grain. Now he must call their attention to 

 certain very interesting facts which resulted from this 

 circumstance of silica so constantly being present in a 

 state of solution. As silica was almost everywhere 

 passing through the rocks in a state of solution, it might 

 constantly be deposited in the rocks, and it had 

 a wonderful power of replacing other sub- 

 stances. Very frequently the remains of plants and 

 animals might be entirely dissolved away, and as por- 

 tions of the plant or animal structure were carried away 

 in solution, particles of silica were deposited, which took 

 the place originally filled by carbonate of lime or other 

 substance, which formed part of the animal or vege- 

 table structure. That was a form of fossilisation. When 

 the materials of the organism were removed and the 

 mineral substance deposited in its place, it was said to be 

 fossilised, and this particular form was called silicifica- 

 tion. Now the wonderful thing was the way in which 

 this silicification went on. For example, in trunks of trees 

 and vegetable objects every portion of carbon, oxygen, 

 and hydrogen which composed the plant tissues might be 

 removed particle by particle, and particles of silica put in 

 their place, and this work might be done so perfectly 

 that the structure retained its original form and all its 

 delicate features. They could imagine it possible that a 

 building like that (the Museum of Practical Geology), 

 which happened to be built of stone, might have every 

 stone taken out and bricks put into their places, and so 

 the stone building would be converted into a brick build- 

 ing. Or they might have the limestone removed stone by 

 stone, and sandstone put in its place. That was what was 

 done in silicification, only the bricks were so minute, and 

 so perfectly did Nature do her work, that when they took a 

 section and examined it with the microscope, they found 

 all the minutest structures reproduced in the silica. [Speci- 

 men shown with lantern, of piece of wood which had been 

 completely silicified.] All the delicate cells and vessels 

 were completely reproduced ; and all the foreign sub- 

 stances which lay in those cells were reproduced in the 

 silica, and if they were to analyse those flints they 

 would find only a minute percentage of the original 

 elements — the carbon, the hydrogen, the oxygen, and 

 the nitrogen. This work of silicification had gone on 

 in such a way that as each molecule of the plant-tissue 

 had been removed a molecule of silica had taken its 

 place, and its whole structure had been reproduced, and 

 all its minutest details were produced with the greatest 

 accuracy. Not only were plants silicified, but so were 

 corals and many other structures. [Specimens of fossils 

 building up some of the varieties of chalk were shown.] 

 Those fossils originally consisted of calcic carbonate, or 

 carbonate of lime. Sometimes a whole mass of chalk 

 was converted into silica. [Specimen of mass of chalk 

 from Scotland, which was full of organisms exactly like 

 those of ordinary chalk, and the whole mass had been 



converted into flint. It was a mass of chalk, in which 

 the process of silicification had gone on to the fullest 

 extent] Sometimes those objects had undergone a 

 remarkable and considerable change. The colloidal 

 silica, of which they were originally formed, had been 

 converted into crystalline silica. Now he had to call 

 their attention to the fact that when they studied a large 

 series of flints they would find that some of those flints 

 were most obviously nothing but chalk-mud, which had 

 been silicified. All the characters that were seen in 

 chalk-mud were found in these particular flints. But 

 as they studied flints they would find a wonderful 

 change went on in them. They would find that 

 the colloidal silica forming the flints had been con- 

 verted into crystalline silica ; they would find that the 

 colloidal silica which had filled up all the interspaces 

 between the organisms had in some cases undergone a 

 similar change, and that the material had assumed a more 

 or less perfectly crystalline character. Thus in borne 

 flint they might recognise chalk mud in which perfect 

 silicification had taken place ; every particle of chalk mud 

 had been removed in a state of solution, and colloidal 

 silica had been put in its place. From that first stage they 

 would trace a perfect series of gradations. They would 

 find minute crystals taking the place of original colloidal 

 silica, and this crystalline character might go on increasing 

 and increasing until at last the whole of the original 

 organic structures were completely obliterated, and there 

 was nothing but a crystalline appearance. He would 

 point out that flints presented too many different ap- 

 pearances. Sometimes flints appeared black, or, at 

 all events, a dark colour. At other times they ap- 

 peared perfectly white. As he pointed out in his la^t 

 lecture, they sometimes found white flint in the interior 

 of black flint, and a layer of white flint on the outside. 

 Generally they found white flint in the interior, with 

 black outside. 



If they took a piece of black and a piece of white flint, 

 and examined them carefully by means of chemical 

 analysis, it would be found that they did not differ in their 

 composition. Black and white flint consisted of silica and 

 nothing but silica ; but the difference between these two 

 substances was entirely a difference of aggregation. A 

 piece of blotting paper was opaque, but put oil on it and 

 itbecametransparent. It was the same with black or white 

 flint. When flints were examined microscopically they 

 would find in the white flint a great number of particles 

 of silica separated by air, but in the translucent black 

 flint colloidal silica had been deposited in the inter- 

 spaces, and they got a continuous light. The oil on 

 the blotting paper had nearly the same effect upon 

 light as the silica has in the interspaces of flint — they 

 got a continuous stratum through which the light 

 passed freely. [Pictures on screen by aid of lantern 

 showing differences which are presented in flints.] 

 He had introduced the last two slides in order to 

 bring before them clearly the distinction of the two 

 forms of silica, and the fact that one kind of silica was 

 constantly found passing into the other kind. They had 

 most of them that evening seen on the screen with the 

 actual eye or with the eye of faith, a sufficient number 

 of these objects to enable them to reason upon them as 

 he hoped to do in their next lecture, in which he would 

 endeavour to show how these facts that they hid been 

 collecting in the four lectures already given would enable 

 them to frame a reasonable theory concerning the origin 

 of flints. 



