CHRONOLOGICAL SUCCESSION OF AQUEOUS ROCKS. 35 



boniferous) coexisting with fishes and other animal fossils 

 which are equally admitted to be characteristic of the later 

 formation of the New Eed Sandstone (Permian). This 

 conjunction of an ancient flora with a more modern fauna 

 occurs in the gas -coals of Bohemia, and much difference 

 of opinion has been expressed as to the proper interpreta- 

 tion to be placed upon the facts. If we regard the plants 

 alone, we must place the beds in question in the Carbon- 

 iferous formation, whereas if we look to the animal remains 

 alone, we should with an equal absence of hesitation refer 

 the strata to the Permian. A still more important case 

 of an essentially similar nature occurs in North America, 

 and concerns the boundary-line between the Cretaceous for- 

 mation and the Tertiary, two groups of rocks which in the 

 Old World are separated by an extraordinarily abrupt and 

 conspicuous line of demarcation. In North America we 

 find a series of rocks which contain unquestioned Cretaceous 

 fossils, and another great group of deposits which are charged 

 with an equally unequivocal series of Tertiary fossils ; but 

 between these there is an immense series of beds some four 

 thousand feet in thickness which contains the remains of 

 undoubted Cretaceous Invertebrate and Vertebrate animals 

 mixed with a vast number of regular and unquestionable 

 Tertiary plants. If we look to the animals, we must place 

 this series (known as the " Lignitic Series," from the presence 

 in it of beds of lignite) in the Cretaceous ; whereas from the 

 evidence of the plants alone we should have to consider it 

 as the base of the Tertiary. Upon the whole, however, and 

 without entering into any detailed discussion of the question, 

 it would appear that in all such cases plants have a much 

 smaller value as tests of the geological position and age of 

 the beds in which they occur than may be justifiably at- 

 tached to the remains of the Marine Invertebrates, while 

 these, again, are inferior in this respect to the remains of 

 Vertebrates. Judging by this canon, in which most author- 

 ities are now agreed, the Bohemian gas-coals must be con- 

 sidered as Permian, and the great Lignitic series of North 

 America must be considered as forming the summit of the 

 Cretaceous series. 



