42 



ains. The Dakota group, resting upon Permian land, or deposited upon its 

 shores, was under the influence of a land climate at the end of a stationary 

 period of long duration. Its climate was accordingly dry, and proportionally 

 cold. A long period of subsidence covered it, to the west at least, by a suc- 

 cession of deep marine formations recorded in the different stages of the Cre- 

 taceous strata. At the opening of the new epoch, the Eocene sandstone, full 

 of fucoidal remains, is brought up to the surface of the sea. The first land 

 that covers it is still, like the low land, under an atmosphere of vapors. The 

 climate is proportionally warm, or at least the mean temperature is higher, 

 and the vegetation appears in exact relation to these atmospheric circum- 

 stances. It is a time for a luxuriant vegetation of ferns, of palms, of the 

 boggy plants which favors the growth of peat, and the heaping of vegeta- 

 ble matter in a succession of beds which have afterward been decomposed 

 into coal. That this vegetation has no relation whatever with that of the 

 Dakota group is a matter of course, as it is born or established under totally 

 different influences and circumstances. The difference between the flora of the 

 Dakota group and that of the formation which I call Eocene is, therefore, 

 attributable to a change of climatic influences, rather than to difference of age, 

 a conclusion admittable for all the changes indicated by geological floras. 

 But on this assertion, and especially in regard to its application to the floras 

 of the Tertiary groups of the West, nothing more can be said until the fossil 

 plants of these formations are published, a work prepared for another volume. 

 I reserve for that time also, the discussion on this new hypothesis : that 

 groups of identical fossils, especially vegetable ones, do not prove or indicate 

 contempoi'aneity of the formations which they characterize, when these form- 

 ations are observed at great distances or under different degrees of latitude. 



§ 9. DESCRIPTION OF SPECIES. 



It is not possible to attach any importance to the priority of nomencla- 

 ture of so-called species of fossil plants, as long as they are known from mere 

 descriptions. The analogy, not to say the specification, of fossil leaves is un- 

 certain enough when the descriptions are illustrated with carefully made 

 drawings or figures which clearly define the essential characters, the outline 

 or general forms, with the nervation of the leaves. In the written records of 

 vegetable fragments, even of whole and well-preserved leaves, the descrip- 

 tions, though exact they may be, are always subjected to erroneous repre- 

 sentations of the mind. For the same reason, I consider not only as a right, 



