350 THE CAUSES OF THE — XI 
of lime and carbonate of lime. Some years ago, 
I had to make an inquiry into the nature of some 
very curious fossils sent to me from the North of 
Scotland. Fossils are usually hard bony structures 
that have become imbedded in the way I have de- 
scribed, and have gradually acquired the nature and 
solidity of the body with which they are associated ; 
but in this case I had a series of holes in some 
pieces of rock, and nothing else. Those holes, 
however, had a certain definite shape about them, 
and when I got askilful workman to make castings 
of the interior of these holes, | found that they 
were the impressions of the joints of a backbone 
and of the armour of a great reptile, twelve or more 
feet long. This great beast had died and got 
buried in the sand; the sand had gradually 
hardened over the bones, but remained porous. 
Water had trickled through it, and that water 
being probably charged with a superfluity of 
carbonic acid, had dissolved all the phosphate and 
carbonate of lime, and the bones themselves had 
thus decayed and entirely disappeared ; but as 
the sandstone happened to have consolidated by 
that time, the precise shape of the bones was 
retained. If that sandstone had remained soft a 
little longer, we should have known nothing what- 
soever of the existence of the reptile whose bones 
it had encased. 
How certain it is that a vast number of animals — 
which have existed at one period on this earth 
