292 GEOLOGICAL SURVEY OF THE TEERITORIES. 



long periods of time aud formations which separate them is an instruc- 

 tive document, whose importance as poiut of comparison in the study of 

 the geographical distribution of our present flora and of its analogy to 

 ancient types will be easily accepted by botanists. But here it has to 

 be considered merely in connection with the question of the age of the 

 Lower Lignitic. 



The Cretaceous Dakota group is separated from Point of Eocks by a 

 thickness of strata about the same as that which is marked between 

 Point of Eocks and Black Butte. Nevertheless, between the floras of 

 the Nebraska and Kansas Cretaceous and that of Point of Eocks and 

 Black Butte, we find scarcely any analogous type, and not a single 

 identical form. The erosions loay have indeed considerably thinned 

 the marine strata representing the Cretaceous above the Dakota group, 

 but that cannot lessen the strength of the deduction made from the 

 total disconnection of two floras, one of which denotes by its essential 

 characters a marked dissimilarity of atmospheric circumstances, a 

 weighty evidence, if not a positive proof, of a change of epoch, if not in 

 the sea, at least upon the land. It is useless to repeat that, as yet, no 

 marine invertebrate remains of Cretaceous type have been discovered in 

 the whole Lignitic measures above Point of Eocks. We may admit, 

 however, that while the Tertiary age was, at its beginning, represented 

 as a land formation, as seen by its flora, a Cretaceous marine fauna may 

 have still locally persisted in deep seas. Facts of this kind are recorded 

 in European geology. The presence of the Saurian Agathaumas in the 

 Lignite bed of Black Butte is then certainly explainable as denoting the 

 wandering of that animal out of its domain, and its death by penetra- 

 ting into a peat'bog and being irretrievably swallowed by its soft mat- 

 ter. If once imbedded in soft peat, no animal, not even man, can get out 

 of it. By this fact, and also from the reason that the coriaceous, ligne- 

 ous plants of the bogs are not food for mammals, I explain the scarcity of 

 bones of Eocene animals in the lower beds of the Lignitic. As a shore 

 formation, a surface covered with deep bogs surrounded by sand wastes, 

 this primitive land would not aiibrd food to mammals or even be accessi- 

 ble to them. Every one who has explored a peat-bog knows how these 

 formations are deprived of animal life. Very few bones of the 

 Aurochs have been found in the bogs of North Germany, and the area 

 covered by the Lignitic shows how compact and continuous, not to say 

 universal, were those swamps of the Lower Tertiary, I believe, there- 

 fore, that if the bones of Eocene mammals are not discovered in the 

 lowest part of the Lignitic, they will be found in the upper strata. 

 Moreover, the agglomeration of bones in certain localities depend on pecu- 

 liar circumstances, and do not immediately and forcibly relate, like plants, 

 to the general characters of a whole period. 



The question of the subdivision of the Lignitic or Tertiary measures, 

 which I have separated in four groujDS, from the non-coincidence in the 

 general character of the flora, is still disputed, and this division con- 

 tradicted by the assertion that the discordance is merely apparent, 

 and a result of the geographical distribution of species, as we may 

 see it now in groups of plants at distant localities. The contempo- 

 raneity of the fossil floras is not merely marked by the identity of some 

 species, but also by a kind of general character denoting the same 

 climatic circumstances. The modification due to the geographical dis- 

 tribution may be easily recognized by the presence or absence of a 

 number of species in the flora of the Bitter Creek basin, of that of 

 Colorado, the Eaton Mountains, the Lower Union group, the Missis- 

 sippi, and Vancouver. There is between these localities a wide dis- 



