256 THE FLORA OF THE DAKOTA GROUP. 



persistence and representation of the types of the Dakota Group up to the 

 Laramie, as cleai'ly as it is observable in this flora and through the different 

 stages of the Tertiary to the present time. 



Ah-eady the evidence obtained from the study of tlie Dakota Group 

 remains warrants the couchisiou that the flora of North America is not at 

 the present epoch, and has not been in past geohigical times composed of 

 foreign elements brought to this continent by migration, but that it is indig- 

 enous. Its types are native; the diversity of their representatives has been 

 produced by physical influences; their affinities, therefore, or the relation of 

 tlieir modific;\tion or derived foiuns can not be looked for in the vegetation 

 of distant countries. This evidence greatly simplifles the researches, and 

 therefore the affinities rest upon more solid ground. These conclusions seem 

 confirmed by the great analogy of climatic circumstances recognized 

 as existing between the characters of the flora of the Dakota Group and those 

 wliich now govern the vegetation of the North American continent. All the 

 plants of the American Cenomanian, except those of Ficus and the Cycads, 

 iniglit find a congenial climate in the United States between 30° and 40° of 

 latitude. Even the exceptions noted above may be omitted; for the growth 

 of some kinds of plants, for example the Cycads, essentially depends on a 

 moderate and liumid climate, witliout striking extremes of temperature, and 

 of circumstances like tliose governing the climate of the southern shores of 

 Fk)rida, wliere species of Ficus still grow luxuriantly. 



A single modification of the character of the vegetation generally fol- 

 lows great geological distui'bances which produce permanent changes in tlie 

 atmospheric conditions of a country. From the base of the Rocky Moun- 

 tains to tliat of the Alleghanies the land surface, with an expanse of more 

 tlian 140,000 square miles, has remained unl)roken by any chain of moun- 

 tains or even by hills <»f moderate altitude, from the begiiming" of the Cre- 

 taceous period. Even the absence of ibift deposits upon a great portion of 

 these vast plains .shows how little the Glacial Period influenced their physi- 

 cal conditions. The result lias been a prolonged uniformity of climate and 

 of course the preservation of the original types of the flora, subjected to 

 some modification of their original characters, witliout destroying them or 

 forcing their removal by the introduction of strange or exotic forms. 



