HO W EXTINCT MONSTERS ARE PRESER VED. 13 



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 illus- 

 trated 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 sohd rock, the im- 

 pressions are fixed imperishably. Examples of these interesting 

 records may be seen at the Natural History Museum at South 

 Kensington. Even soft jelly-fishes have left their mark on certain 

 rocks ! At a place in Bavaria, called Solenhofen, there is a 

 remarkably fine-grained limestone containing a multitude of 

 wonderful impressions. This stone is well known to lithographers, 

 and is largely used in printing. On it the oldest known bird has 

 left its skeleton and faithful impressions of its feathers. 



The footprints of birds and reptiles are by no means un- 

 common. Such records are most valuable, for a great deal may 

 be learned from even a footprint as to the nature of the animal 

 that made it (see p. 79). 



Since the greater number of animals described in this book are 

 reptiles, quadrupeds, and other inhabitants of the land, and only 

 a few had their home in the sea, we must endeavour to try and 

 understand how their remains may have been preserved. Our 



