i 4 EXTINCT MONSTERS 



that we find fossils least changed from their original state ; 

 for time works great changes, and too little time has elapsed 

 since those periods for any considerable alterations to have 

 taken place. But when we come to examine some of the earlier 

 rocks, which have been acted upon in various ways for long 

 periods of time, such as the pressure of vast piles of overlying 

 rocks, and the percolation of water charged with mineral sub- 

 stances (water sometimes warmed by the earth's internal heat), 

 then we may expect to find the remains of the world's lost 

 creations in a much more mineralised condition. Every fossil- 

 collector must be familiar with examples of changes of this 

 kind. For instance, shells originally composed of carbonate of 

 lime are often found to have been turned into flint or silica. 

 Another curious change is illustrated in the case of a stratum 

 found in Cambridgeshire and other counties. In this remarkable 

 layer, only about a foot in thickness, one frequently finds bones 

 and teeth of fishes and reptiles. These, however, have all 

 undergone a curious change, whereby they have been converted 

 into phosphate of lime a compound of phosphorus and lime. 

 It abounds in " nodules," or lumps, of this substance, which 

 along with thousands of fossils, are every year ground up and 

 converted by a chemical process into valuable artificial manure 

 for the farmer. 



The soft parts of animals, as we have said before, cannot be 

 preserved in a fossil state ; but, as if to compensate for this loss, 

 we sometimes meet with the most faithful and delicate impres- 

 sions. Thus, cuttle-fishes have, in some instances, left, on the 

 clays which buried them up, impressions of their soft, long arms, 

 or tentacles, and, as the mud hardened into solid rock, the im- 

 pressions are fixed imperishably. Examples of these interesting 

 records may be seen at the Natural History Museum at South 



