FROM THE TERTIARY OF THE STATE OF MISSISSIPPI. 



425 



marked, they cannot be taken, I think, as conclusive indication of a difference in the 

 horizontal position of the strata. At least not yet, when the materials for comparison arc 

 so scanty. Changes in the vegetation are often merely local, and caused by casual cir- 

 cumstances. For example, swamps along water-courses, have preserved in their clay 

 deposits, water or shore plants, either fallen from trees grown in the same place, or trans- 

 ported by currents ; while swamps in the forest, though contemporaneous, have in their 

 clay deposits true forest leaves of a different character from those of the former. More- 

 over, the floras of the recent formations, principally of the Tertiary, have been far more 

 local or grouped than they were in the old formations, especially during the coal epoch, 

 whose vegetation is of the widest general uniformity. Even our red shale, though the 

 plants which it contains have; the same general facies, indicate specific differences of the 

 same kind. For, in the specimens of Sommerville, Tennessee, and those of Tippah, Mis- 

 sissippi, there is only one species, Querents Lyellii, common to both. It is the leaf, which 

 from a broken specimen, and from its likeness with our now living Laurus Caroliniensis, 

 I have previously considered as identical with it * 



A second question, more important still, concerns the age of the formation where our 

 fossil leaves have been found, as indicated by the species here described. It must be ad- 

 mitted, at first, that the attempt to draw some positive deductions on this subject, with 

 the small amount of materials at present collected, and without any point of comparison 

 from our American formations, may result in unreliable conclusions. For we know 

 nothing yet of the fossil flora of our mesozoic time ; even the line of separation between 

 the vegetation of the Tertiary and that of the Cretaceous is not traced. And if we look 

 for comparison to what is known of the fossil plants of Europe, we find there with short 

 memorials of Eoccnic and Cretaceous floras, a splendid record of Miocenic vegetation, -j- 

 where most of the types of our actual flora are represented together with species analo- 

 gous to some which are recognized as belonging to the American Cretaceous. It is not 

 surprising that a comparison made with such materials is liable to error. Yet, this is not 

 a reason why we should lay aside our fosnil plants with distrust. On the contrary, it is 

 important to examine them carefully now and to begin by recording facts and data for 

 future use. For, as the geological position of the group where they have; heen collected 

 is well ascertained, these botanical remains, once collected in number and well studied, 

 may furnish a point of comparison for future discoveries. 



The horizon of this group, winch we call, with Prof. Hilgard, North Lignitic, is marked 

 first by a section by this eminent geologist, of what he considers the tertiary strata of 

 Mississippi. 



* American Journal of Science, § 2, vol. xxvii, page 863. 

 | Especially in the admirable works of Heer. 



vol. xm. — 54 



