420 GEOLOGICAL SURVEY OF THE TERRITORIES. 



very simple organization, or, so to say, in an adventive state of develop- 

 ment, prepared in that way to rapidly undergo a series of modifications 

 under every kind of physical influences. There is as yet scarcely any 

 document in confirmation of this hypothesis and still less in contradic- 

 tion of it. It is, in any way, adaptable to the explanation of some pecu- 

 liar analogies remarked in the characters of the geological floras. 



The vegetation of the Dakota group has a distant relation to that of 

 the Upper Cretaceous flora of Europe by identity of a few of its species, 

 especially ferns. But, as yet, little is known of the succession of the 

 vegetable groups during the European Cretaceous, and of the relations 

 of plants to the geological divisions of that epoch ; and though the an- 

 alogies may become more marked by future discoveries or publications, 

 it is only from the data furnished by the American Cretaceous flora that 

 we can get some kind of criterion of the climatic circumstances which 

 have marked its general characters.*' The descriptions of the species 

 of this flora and the details in regard to their relation, as published in 

 our flora of the Dakota group,t evidently show its relation to a moderate 

 climate, about of the same average degree as that of the middle region 

 of North America. A number of Cretaceous genera are still represented 

 in our arborescent vegetation. 



From the Dakota group upward, there is no trace of land-vegetation 

 in the whole North American continent until we reach the Lower Lig- 

 nitic formation. All the intermediate strata are marine, and the series 

 of animal remains, which they have preserved in great abundance, are 

 uninterrupted and uninterruptedly Cretaceous in their characters as 

 high as the Lignitic. Animal Cretaceous remains have been found, 

 as remarked formerly, even in shale overlayiug Lignitic deposits. 

 Now, in comparing fossil plants of the first or lowest group of the 

 Lignitic, we should expect to find, merely considering its immediate 

 succession to strata of Cretaceous age, a flora with some distinct anal- 

 ogy to that of the Dakota group : most of its genera, some of its 

 species, too. But it is not the case. Some genera, of course, are repre- 

 sented in both floras, but by different types ; and they do not have any 

 identical species, nor even any closely-related forms. There is in the 

 general character a kind of related fades ; but specific types of the 

 former floras seem to have been destroyed during the prevalence of the 

 marine Cretaceous period, and above its fucoidal sandstone, even within 

 its upper strata, and in connection with Lignitic deposits, there appears 

 a new flora without positive relation with former vegetable types, 

 and with but few of those of subsequent groups of plants or younger 

 geological floras. This anomaly may be explained in two ways ; either 

 by supposing that during the prevalence of the marine formations, 

 or by the submersion of the land, all the genera and species of the 

 Cretaceous have been annihilated, and that a new generation of vegeta- 

 ble types has covered the new land as fast as it appeared above the 

 surface of the water; or that during the period of the marine Creta- 

 ceous, the climate has been gradually modified, and that, therefore, the 

 laud at its first apparition has been invaded by a vegetation in harmony 

 with the climatic circumstances governing this new epoch. This last 

 supposition seems the only one admittable, the more so as it does not 

 consider the hypothesis of a general destruction of vegetable types and 

 of subit renovation of others or of the creation of a new vegetable world. 



* Two memoirs of the Lower and Upper Cretaceous floras of Greenland have been 

 prepared and are now in the way of publication by Profe8.sor Heer. 



t Memoir on the fossil plants of the Cretaceous Bakota group of the United States, 

 (1874.) 



