j6 INTRODUCTION. 



them of water holding carbonic acid in solution. By this means 

 fossils of a calcareous nature are dissolved out of the rock, and may 

 leave no traces behind. This cause, however, can only operate to 

 any extent in more or less loose and porous arenaceous deposits. 



Lastly, " cleavage " may be mentioned as a common cause of the 

 disappearance of fossils. But the cleavage must be very intense, 

 if it actually prevents the recognition of the deposit as one in 

 which fossils formerly existed, though cases are not uncommon in 

 which this occurs through thousands of feet of strata. As a more 

 general rule, however, it is not very difficult to determine whether a 

 cleaved rock has ever contained fossils or not, though it may be 

 quite impossible to make out the exact nature and character of the 

 organic remains. 



