American and English Fossil Floras. 499 



bed, was characterized by a fauna in some degree special to it, and 

 which kept up with it as it travelled farther and farther landward. 

 In considering a time so remote as the Cretaceous, when climates 

 partly depended on the internal heat derived from the cooling 

 earth, we may disregai'd the influence of latitude on distribution, 

 and assume that like conditions of depth would furnish like faunas. 

 The faunas of these ever-travelling zones of depth of the old 

 Cretaceous sea would resemble each other almost to the point of 

 identitj', so long as the character of the matrix remains unchanged, 

 and we must therefore be prepared to find the greatest similarity 

 between the fossils obtained from the same zone, at any interval 

 of distance, and not conclude therefrom that they lived synchronously 

 all along the line. They might differ very much from the zones 

 above and below, which were deposited at different depths, and 

 maintain their distinctness over the most extensive areas, as Dr. 

 Barrois has ascertained that they in fact do. 



The time required to accumulate a mass of sediment, composed 

 mainl}' of minute organisms, the relics of which equal 1200 or 1400 

 feet in vertical thickness, added to the immensely increased time 

 demanded by its travelling progress from Kent to the Crimea, must 

 have been so enormous, that during its lapse the natural law of the 

 evolution of organic forms would have produced changes in animal 

 and vegetable life, a progress in the great chain of causes and 

 effects, which should be appreciable to any competent investigator. 



We have noticed that the Neocomians and Gault of England and 

 Western France contain a very varied and considerable flora, repre- 

 sented by foliage and fruits, without, however, affording the slightest 

 trace of the presence of angiospermous dicotyledons. Even the Grey 

 Chalk and Blackdown Greensand have only yielded Conifers and a 

 WilUamaonia of Jurassic type. We cannot account for their absence 

 by supposing that our area was isolated, since in the immediately 

 preceding Wealden period neither its fauna nor flora differed in any 

 respect from that of the rest of Europe. But when we reach countries 

 as distant as Limburg, Saxony, and Bohemia, we are confronted with 

 a richly dicotyledonous flora, underlying a precisely similar sequence 

 of Cretaceous deposits; comprising Greensands, Chloritic marl. Chalk 

 with and without flints ; to that underlaid in England by an almost 

 Jurassic flora. Such a fact remained until now completely unaccount- 

 able ; but on the present theory it affords no ground for surprise. 

 Satisfactory as this corroboration is, it by no means stands alone. It 

 further indicates, unless we are prepared to deny in the case of 

 dictyledons the otherwise unvarying law, and definite order, of evolu- 

 tion, how great an interval was I'equired for the Chalk deposit to travel 

 a distance of only 300 to 400 miles — an interval great enough to have 

 permitted enormous progress in the evolution of dicotyledons. 



Leaving plants for the present, and turning to the higher Verte- 

 brata, we find that in Europe they afford us hardl}^ any assistance. 

 Mosasaurus first appears in the Chalk in England as an exceedingly 

 rare fossil, and its preponderance over other reptilia in the Maestricht 

 limestone, and in other newer deposits, is, I believe, an indication of 



