HO IV 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 solid 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 
