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Se a ee ee 
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 
favourable 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 kad commenced during life to 
seerete spicules from the silica in the water, there would be as it 
‘were, a starting point for the precipitation of more silica. It is 
well known for example, that when a dissolved substance does not 
readily crystallize, it can be made to do so by dropping in a small 
erystalline 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 silica 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. 
