BLUE MUD. 73 



while to the whole group of secondary and 

 tertiary strata, comprising almost all the best- 

 known English formations red marl, lias, 

 oolite, greensand, chalk, eocene, miocene, 

 pliocene, and drift we can only give a single 

 aeon to be divided between them. Such facts 

 will sufficiently suggest how comparatively 

 modern are all these rocks when viewed by 

 the light of an absolute chronology. Now, 

 the first fishes do not occur till the Silurian 

 that is to say, in or about the seventh aeon 

 after the beginning of geological time. The 

 first mammals are found in the trias, at the 

 beginning of the tenth aeon. And the first 

 known bird only makes its appearance in the 

 oolite, about half-way through that latest 

 period. This will show that there was plenty 

 of time for their development in the earlier 

 ages. True, we must reckon the interval 

 between ourselves and the date of this blue 

 mud at many millions of years ; but then we 

 must reckon the interval between the lias and 

 the earliest Cambrian strata at some six times 



