17 



extent the two elements are interchangeable; one can take the place 

 of the other in a compound. Is it not probable then that the 

 carbonic acid given off by the decaying body of a sponge or other 

 animal, may change places with the silicic acid which is in combi- 

 nation with other substances in the water, and that where we had 

 a silicate of soda or potash in the latter we now get a carbonate, 

 while silica from the silicic acid deposits itself on the remaining 

 organic matter in its place? The operation must have gone on 

 gradually and under very favourable circumstances, the replacement 

 must have been by molecules since you see the form of the shell or 

 the sponge so perfectly preserved. And so rarely would these 

 favom-able circumstances occur, that we may be quite sure a thou- 

 sand sponges "died and left no sign" for each one that has left 

 its own monument behind. 



And in the case of a sponge which had commenced during life to 

 secrete spicules from the silica in the water, there would be as it 

 were, a starting point for the precipitation of more silica. It is 

 weU known for example, that when a dissolved substance does not 

 readilv crystallize, it can be made to do so by dropping in a small 

 crystalline fragment, around which as a nucleus precipitation imme- 

 diately commences. But of course this was occasional. It was not 

 always round a sponge ; we get shells wholly converted into flint, 

 and in perhaps the majority of the stones we find no organism at 

 all. The flat cakes of flint that form a striking feature in the chalk 

 cliffs seldom produce any. Yet we are entitled to believe that, one 

 and all, their formation was started by organic matter. Life in 

 ten thousand forms completely covers enormous spaces in some 

 parts of the bed of the ocean, and these might be in an advanced 

 stage of decomposition before the sihca gathered over them. Thus 

 all definite traces of them would be lost, except the dark colour of 

 the flint caused by the carbonaceous matter. 



Flint does not occur invariably in lines, and occasionally the lines 

 are not horizontal but the silica appears to have filled up transverse 

 fissures in the chalk. But solitary flints as might be expected occur 

 scattered through the rock, and it is these generally which reach a 

 huge size. 



The lecture was illustrated by a large collection of specimens, 

 including various common flints, fossils in flint, other forms of 

 silica, Euplectella, &c,, as well as microscopic specimens shown in 

 instruments lent by some of the members. 



