228 THE FLOliA OF THE DAKOTA GliOUl'. 



or shady groiiiul, oi* upon the slopes or thy summits of mountains l)atluMl 

 by tho clouds, or aloug shores Avhere they <u'e coustaiitly or periodically 

 moistened by the misty winds of the ocean. In the older geological times 

 the earth, wliose heated crust by contact with water caused constant and 

 jjrodigious vaporization, was surrounded by a tliick vail of vapors impen- 

 etrable to the ra}'s of the sun. This rendered the climatic conditions unal- 

 terable, not only locally but over the whole surface of the globe. Hence 

 the uniformity and peculiar character of the vegetation of the Devonian 

 and Carboniferous ages, composed, as they are, of scmi-acjuatic or amphib- 

 ious woody plants of gigantic size, such as Ferns and L}'copods. In the 

 floras of the subsequent ages, first the Permian for example, the Conifei's 

 gradually become more abundant; then in the Triassic and Jurassic'thc 

 Cycads increase in the number of their representatives, and thus the vege- 

 tation shows only the slow and gradual modification of some of its predom- 

 inant groups. But until the beginning of the Cretaceous, the ^•ariations 

 relate to certain specific or generic forms, Ijut do not affect the general 

 chai'acter of the vegetation of the world, being still under the influence of 

 a superabundant atmospheric humidity. The vegetable remains are always 

 Acrogens, the Fems especially being the predominant and less diversified 

 component of the ancient floras, with the Conifers and the Cycads next in 

 order. And even in the Wealden, which closes the Jm-assic period, the 

 plants, as far as they are known by fossil remains, are still referable to the 

 same groups of plants as those of the Jurassic. It is only from the base 

 of the Cretaceous that tlie vegetable remains show by certain characters a 

 marked diminution in tlie influence of atmospheric humidit}-. Then the 

 thickened crust of the earth had gradually become more cooled; the misty 

 atmosphere was clearer and allowed the ravs of the sun to penetrate and 

 act directly upon the surface of the earth, which sei*ved to bring out local 

 or peiiodicid alterations of climates, conditions, and seasons, and tluis to 

 force under this new influence and b}- its action important changes in the 

 nature and aspect of the vegetation of the world, first by the introduction 

 of tlie Monocotyledons and soon after by that of Dicotjledons. 



Perhaps, from the presence of one monocotyledonous species in tlie 

 flora of the Wernsdorf shale (Neocomian) and one of dicotydonousleaf and 

 of five Monocotyledons in the fomiation of Konie (Urgonian), from the 

 great decrease in the juunliers of Cryptogams and Coinfers and the pro- 

 digious multiplicjition of Dicotvledons in the schists of Atane (Cenomanian) 

 innnediately superposed ujion those of Kome, and in tlie forjuatiou of the 



