470 PALEOBOTANY. 



the highest Cretaceous and the lowest Tertiary sediments — 

 a break marked not only by a universal unconformity, but 

 also by a great change in the characteristic fauna of the two 

 deposits. On the other hand, in North America the highest 

 unquestioned Cretaceous beds (the marine deposits of the 

 Fox-Hills group) are succeeded by a great series of strata, 

 well known as the Fort Union or Great Lignite series, the 

 true stratigraphical position of which has been the subject 

 of much dispute. These deposits consist of nearly four 

 thousand feet of sandstones, shales, and beds of lignite, 

 which rest quite conformably upon the unquestioned Cre- 

 taceous deposits of the Fox-Hills group below, and which 

 are succeeded unconformably by unquestioned beds of Ter- 

 tiary age. In their lower portion they contain a number 

 of marine organic remains, but these gradually disappear as 

 we ascend in the series, and its upper portion is generally 

 characterised by the remains of land and fresh-water shells, 

 associated with a vast abundance of vegetable fossils, chiefly 

 of the nature of detached Dicotyledonous leaves. The dif- 

 ficulty of the problem as to the real age of this great and 

 remarkable deposit arises chiefly from the fact that its marine 

 fossils are fundamentally of a Cretaceous type, whilst the 

 remains of plants have an equally distinct Tertiary facies. 

 Thus we find such characteristic Cretaceous Mollusca as 

 Inoceramus, Ammonites, and Baculites, with unquestionable 

 Dinosaurians (AgatJiaumas), side by side with a luxuriant 

 flora of an essentially Tertiary aspect, comprising such mod- 

 ern genera as Quercus, Acer, Popuhis, Ulmus, Moms, Fagus, 

 Juglans, Alnus, Corylus, Ilex, Platanus, Ficus, Cinnamomu'm, 

 Smilax, Laurus, Bhamnus, Magnolia, Facalyptus, Thuja, 

 Sequoia, Ahies, Taxodium, Sahal, &c. Whilst this associa- 

 tion of Cretaceous animals with Tertiary plants is undoubted, 

 much difference of opinion obtains as to how it ought to be 

 interpreted. On the one hand, high authorities, such as Dr 

 Heer, and Professors Lesquereux and Dana, are of opinion 

 that the i:>lants ought to carry the day, and that the Lignitic 

 Group ought to be considered as the base of the Tertiary 

 series. On the other hand, equally high authorities, such as 



