IMPERFECTION OF GEOLOGICAL RECORD 259 



instance, the several species of the Chthamalinaj (a 

 sub-family of sessile cirripedes) coat the rocks all over 

 the world in infinite numbers : they are all strictly 

 littoral, with the exception of a single Mediterranean 

 species, which inhabits deep water and has been found 

 fossil in Sicily, whereas not one other species has 

 hitherto been found in any tertiary formation : yet it 

 is now known that the genus Chthamalus existed during 

 the chalk period. The molluscan genus Chiton offers 

 a partially analogous case. 



With respect to the terrestrial productions which 

 lived during the Secondary and Palaeozoic periods, it 

 is superfluous to state that our evidence from fossil 

 remains is fragmentary in an extreme degree. For 

 instance, not a land shell is known belonging to either 

 of these vast periods, with the exception of one species 

 discovered by Sir C. Lyell and Dr. Dawson in the 

 carboniferous strata of North America, of which shell 

 several specimens have now been collected. In regard 

 to mammiferous remains, a single glance at the 

 historical table published in the Supplement to Lyell's 

 Manual, will bring home the truth, how accidental and 

 rare is their preservation, far better than pages of 

 detail. Nor is their rarity surprising, when we re- 

 member how large a proportion of the bones of tertiary 

 mammals have been discovered either in caves or in 

 lacustrine deposits ; and that not a cave or true 

 lacustrine bed is known belonging to the age of our 

 secondary or palaeozoic formations. 



But the imperfection in the geological record mainly 

 results from another and more important cause than 

 any of the foregoing ; namely, from the several forma- 

 tions being separated from each other by wide intervals 

 of time. When we see the formations tabulated in 

 written works, or when we follow them in nature, it is 

 difficult to avoid believing that they are closely con- 

 secutive. But we know, for instance, from Sir R. 

 Murchison's great work on Russia, what wide gaps 

 there are in that country between the superimposed 

 formations ; so it is in North America, and in many 



