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 a skilful workman to make castings 

 of the interior of these holes, I 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 



