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into combination with various substances. It is the unstable 

 form of silica ; the crystalline form is the stable one and is quite 

 insoluble. Opal is a combination of this jelly-like silica with 

 water. The silica of sponges contains about seven per cent, of 

 combined water. In nature we find colloid silica passing into the 

 crystalline by an infinite series of gradations ; chalcedony is one 

 of these forms. 



Now let us glance at these pieces of silicified wood obtained 

 either from the chalk in the neighbourhood or the beach. Note 

 especially this mass of translucent chalcedoay in which the form 

 of every fibre, and of every vessel is accurately preserved. The 

 woody substance itself has gone, it has been replaced by silica. 

 What are the conclusions forced upon us by such a fact ? First, 

 that the silica must have been dissolved in some fluid which was 

 able to penetrat3 into the interior of the wood. Next that a chemical 

 combination must have taken place between the silica, or the 

 solution in which the silica was dissolved, and the substance of 

 the wood, so that the silica took the place atom by atom of the 

 woody fibre. In some of the other specimens of silicified wood 

 the process is in a measure seemingly incomplete. 



There are many points in this curious history of the trans- 

 formation of a piece of wood into a mass of chalcedony still 

 enshrouded in mystery. But the fact still remains that the 

 replacement has taken place. It is a chemical process, and, as I 

 have said, all the steps of ifc are not quite clear to us. Bat silica 

 replaces other things besides wood. Here is a piece of coral from 

 Barbadoes gradually being converted into chalcedony. And 

 Professor Rupert Jones is not without some justification in making 

 replacement the basis of his theory of the formation of flints. 

 " Flint," he says, " is a siliceous pseudomorph of the chalk mud 



. . the fine calcareous detritus which filled the internal 

 tubules of sponges and the cavities of sea-urchins has been 

 changed atom by atom into exquisite siliceous casts of such hollow 

 interior." 



Here for instance is the shell of an echinus or sea-urchin 

 entirely filled with flint. The silica has replaced the organic 

 matter of the body of the animal, but has left the shell itself 

 practically untouched. The carbonate of lime of which the shell 

 consists is in a form evidently which offers more resistance 

 to the chemical reactions necessary in any replacement. In 

 another example the test has disappeared but the delicate markings 

 of the interior are faithfully copied on the flint which filled it. 

 Carbonate of lime, when in the form of aragonite, is seldom or 



