INTRODUCTION. 



organic remains from originally fossiliferous deposits is the 

 percolation through 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. The cleavage, however, 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. 



