Cretaceous in the Northwest. 7 



tion not possible in the Old World. From the Eocene 

 downward, the remains of land animals and plants are found 

 only in lake basins occupying the existing depressions of 

 the land, though more extensive than those now remaining. 

 It must also be borne in mind, that the great foldings and 

 fractures of the crust of the earth which occurred at the 

 close of the Eocene, and to which the final elevation of such 

 ranges as the Alps and the Eocky Mountains belongs, per- 

 manently modified and moulded the forms of the continents. 



These statements raise, however, questions as to the pre- 

 cise equivalence in time of similar floras found in different 

 latitudes. However equable the climate, there must have 

 been some appreciable difference in pi-oceeding from north 

 to south. If, therefore, as seems in every way probable, 

 the new species of plants originated on the Arctic land and 

 spread themselves southward, this latter process would 

 occur most naturally in times of gradual refrigeration or of 

 the access of a more extreme climate, that is, in times of the 

 elevation of land in the temperate latitudes, or conversely, 

 of local depression of land in the Arctic, leading to inva- 

 sions of northern ice. Hence, the times of the prevalence 

 of particular types of plants in the far north would precede 

 those of their extension to the south, and a flora found fossil 

 in Greenland might be supposed to be somewhat older than 

 a similar flora when found farther south. It would seem, 

 however, that the time required for the extension of a new 

 flora to its extreme geographical limit, is so small in com- 

 parison with the duration of an entire geological period, 

 that, practically, this difference is of little moment ; or at 

 least does not amount to antedating the Arctic flora of a 

 particular type by a whole period, but only by a fraction 

 of such period. 



It does not appear that, during the whole of the Creta- 

 ceous and Eocene periods, there is any evidence of such 

 refrigeration as seriously to interfere with the flora, but 

 perhaps the times of most considerable warmth are those 

 of the Dunvegan group in the Middle Cretaceous and those 

 of the later Laramie and oldest Eocene. 



It would appear, that no cause for the mild temperature 



