PAL^ONTOLOGICAL RECORD. 391 



"V. Dlscqjpearance of Fossils : — The last subject ■wliicli need be 

 mentioned in connection with, tbe imperfection of tlie palseontological 

 record, is the subject of the disappearance of fossils from rocks origi- 

 nally fossiliferous. This, as a inile, is due to " metamorphism ;" that 

 is to say, the subjection of the rock to an amount of heat sufficient 

 to cause a re-ai-rangement of its particles. When of at all a pro- 

 nounced character, the result of metamorphism is invariably the 

 obliteration of any fossils which might have formerly existed in the 

 rock. To this cause must be set down many great gaps in the 

 palseontological record and the loss of much valuable fossil evidence. 

 The most striking example which can be given of this is to be found 

 jji 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 fossils 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 in loose and 

 porous arenaceous deposits. 



Lastly, cleavage may be mentioned as a cause of the disappearance 

 of fossils. The cleavage, however, must be very intense, if it actually 

 prevents the recognition of the fossiliferous nature of deposit; 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 chai'acter of the organic remains. 



