49 



foraminifera have yet been detected in the overlying eocene beds. 

 The explanation of this is that the break in the life of these two 

 periods represents an incalculable lapse of time. The cretaceous area 

 was elevated and its fauna emigrated ; when it was again depressed the 

 lapse of time was so great that the life which immigrated then from 

 neighbouring seas was composed of new forms. Indeed, the eroded 

 character of the cretaceous rock upon which the tertiary was laid down 

 would in itself prove the great lapse of time. 



But " the imperfection of the Geological Record" accounts for only 

 some of the causes of " imperfection of the PaUeontological Record," 

 for, if the series of sedimentary rocks had been preserved to us in its 

 entirety and open to our inspection, there won! I yet be the deficiencies 

 owing to (1) the facility with which different animals may be preserved 

 as fossils, (2) the liability to be deposited where they may be preserved 

 and, finally, (3) the liability to be obliterated or destroyed after being 

 deposited. 



To the varied facility with which different animals may be pre 

 served as fossils, enormous deficiencies in the pal peon tological record are 

 due. In the polyzoa, ccelenterata, anneloida and annulosa a large pro- 

 portion, comprising entire classes possess no hard parts, and consequently 

 are unrepresented as fossils, and even in the mollusca and vertebra ta 

 some families are lost to us through the same cause. Birds, owing to 

 their lightness, float after death on the water until devoured, and 

 mammals, the majority of which live on land, have fewer opportunities 

 of being buried in aqueous accumulations, consequently are not so often 

 represented as those forms which are essentially marine. 



In addition to these is the disappearance of fossils from rocks 

 originally fossiliferous. Metamorphism or the subjection of the rock t~) 

 a sufficient heat to cause rearrangement of the particles, and conse- 

 quently an obliteration of the fossils, is the chief cause to which we 

 have to look for the irreparable loss of an enormous mass of palseonto- 

 logical evidence. The life of the great Laurentian series of rocks com 

 prising 30,000 feet (say 6 miles) in thickness of sediments, has been 

 entirely blotted out by this cause. Another cause of obliteration is the 

 percolation of wa ling carbonic acid (rain water, for instance) 



through sand or loose rock. 



