IMPERFECTION OF PAL^ONTOLOGICAL RECORD. 39 



another allied species would not, we cannot at present account 

 for this sudden disappearance of one species and its seeming 

 immediate replacement by another. We may be sure, however, 

 that we have here an imperfection of the palaeontological record, 

 and that in reality any two zones must have been separated by 

 a long period, in which one species became extinct, or was so 

 far modified as to appear as a new species. 



V. DISAPPEARANCE OF FOSSILS. The last subject which 

 need be mentioned in connection with the imperfection of the 

 palseontological record is that of the disappearance of fossils 

 from rocks originally fossiliferous. This, as a rule, is due to 

 " metamorphism " that is to say, the subjection of the rock to 

 a sufficient amount of heat to cause a rearrangement of its 

 particles. When of at all a pronounced character, the result 

 of metamorphism is invariably the obliteration of any fossils 

 which might have been originally present in the rock. To this 

 cause must be set down many great gaps in the palaeonto- 

 logical record, and the irreparable loss of much fossil evidence. 

 The most striking example which is to be found of this is the 

 great Laurentian series, which comprises ,some 30,000 feet of 

 highly metamorphosed sediments, but which, with one not 

 absolutely certain exception, has as yet yielded no remains of 

 life, though there is strong evidence of the former existence in 

 it of fossils. 



Another not uncommon cause of the disappearance of 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 con- 

 tained fossils or not, though it may be quite impossible to 

 make out the exact nature and character of the organic 

 remains. 



