Ch. XVII.J FLORA OF UPFER CRETACEOUS PERIOD. 335 



mains and the geological position of the strata prove distinctly that in 

 the neighborhood of Aix-la-Chapelle, a gulf of the ancient cretaceous 

 sea was bounded by land composed of Devonian rocks. These rocks 

 consisted of quartzose and schistose beds, the first of which supplied 

 white sand and other argillaceous mud to a river which entered the 

 sea at this point, carrying down in its turbid waters much drift-wood 

 and the leaves of plants. Occasionally, when the force of the river 

 abated, marine shells of the genera Trigonia, Turritella, Pecten, &c, 

 established themselves in the same area, and plants allied to Zostera 

 and Fucus grew on the bottom. 



Before the cretaceous flora of Aix-la-Chapelle was known, a few 

 leaves of a dicotyledonous and angiospemious genus called " Cred- 

 neria," were known in the " Quader Sandstein " and " Planer Kalk," 

 of Germany, rocks corresponding in age to the white chalk and gault 

 of England. But such fossil plants were the only representatives in 

 rocks older than the Eocene period of those Exogens which now con- 

 stitute three-fourths of the living vegetation of the globe. 



M. Adolphe Brongniart, when dividing the whole fossiliferous series 

 into three groups in reference solely to fossil plants, has named the 

 primary strata "the age of acrogens;" the secondary, exclusive of the 

 cretaceous, " the age of gymnosperms ; " and the third, comprising the 

 cretaceous and tertiary, " the age of angiosperms." He considers the 

 cretaceous flora as displaying a transitional character from that of a 

 secondary to that of a tertiary vegetation. Conifer ce and Cycadece (or 

 Gymnogens) still flourished, as in the preceding oolitic and triassic 

 epochs ; while, together with these, some well-marked leaves of dico- 

 tyledonous angiosperms appeared. But now that the fossil plants of 

 Aix-la-Chapelle are with certainty referred to an Upper Cretaceous 

 era, the line dividing the ages of gymnosperms and of angiosperms 

 seems to run between the Lower and Upper Cretaceous formations, or 

 between the Lower Greensand and the sand of Aix. 



The resemblance of the flora of Aix-la-Chapelle to the tertiary and 

 living floras in the proportional number of dicotyledonous angiosperms 

 as compared to the gymnogens, is a subject of no small theoretical in- 

 terest, because we can now affirrn that these Aix plants flourished be- 

 fore the rich reptilian fauna of the secondary rocks had ceased to exist, 

 The Ichthyosaurus, Pterodactyl, and Mosasaurus were of coeval date 

 with the oak, the walnut, and the fig. Speculations have often been 

 hazarded respecting a connection between the rarity of Exogens in the 

 older rocks and a peculiar state of the atmosphere. A denser air, it 

 was suggested, had in earlier times been alike adverse to the well be- 

 ing of the higher order of flowering plants, and of the quick-breath- 

 ing animals such as mammalia and birds, while it was favorable to a 

 cryptogamic and gymnospermous flora, and to a predominance of rep- 

 tile life. But we now learn that there is no incompatibility in the co- 

 existence of a vegetation like that of the present globe, and some of 

 the most remarkable forms of the extinct reptiles of the age of gymno- 

 SDerms. 



