41 



able to see, at two horizons of the same formations, the respective floras de- 

 noting so great a difference in the atmospherical circumstances which have 

 governed the distribution of each. 



It seems evident, from what has been formerly said of the grouping of 

 leaves of the same kind upon separate and limited areas of the Dakota group, 

 as also of their generally flat position in the sandstone or upon the shale, that 

 the trees from which the fossil remains are derived have grown in the same 

 localities where the fossil leaves are found. These leaves cannot have been 

 carried down torrents and rivers from mountains whose altitude could account 

 for the climatic differences. They do not either represent a kind of trans- 

 portation by floating islands, like those which are carried down the Amazon 

 River and sunk near its mouth ; they are true representatives of the climate 

 of that epoch. In regard to the Cretaceous floras of Greenland, and to the 

 atmospheric modifications indicated by their characters, we can as yet say 

 very little. The facts and their causes will have to be discussed by the cele- 

 brated author who has had under examination the specimens representing 

 the two groups of plants. And we may be certain that the question will be 

 fully considered by him. I shall be permitted, however, to present these few 

 remarks: 1st. Admitting the relation of the flora of the Dakota group with 

 that of the upper Cretaceous of Greenland, a relation which, though recog- 

 nized as yet by generic affinity only, appears sufficiently close to authorize 

 the conclusion that both have been affected by identical conditions of tem- 

 perature, supposing, also, a contemporaneity of the formations, we might 

 easily account for this relation by the well-known fact that the isothermal 

 zones are wider in proportion to the age of the formations, a fact result- 

 ing mostly from a greater proportion of atmospheric humidity. The same 

 reason has been surmised already for explaining the identity of a number of 

 species of the Western Tertiary, especially of the lignitic measures of Carbon 

 and Evanston, with those of the Miocene of Greenland. 1 2d. The cause of 

 the modifications of climate, either slow and continuous during a period of 

 time, or remarked from different formations of the same epoch, result from the 

 changes of land surface which modify in a corresponding degree the intensity 

 or direction of the elements which enter' into the composition of the atmos- 

 phere. A case serving to illustrate this, is remarked in passing upward 

 from the flora of the Dakota group to that of the Eocene of the Rocky Mount- 



1 Dr. F. V. Haydeu's Report, 1871, p. 312. 

 6 L 



