14 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 hme. 
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 
