of the Lafayette Formation. 397 



known, overlies the latest Tertiary of the Southwest, the 

 Grand Gulf. 



The concrete question whether the Natchez gravel, bearing 

 crystallines, is continuous with, and equivalent to the abundant 

 gravel beds of the adjacent uplands, should be solvable with- 

 out any serious difficulty by direct tracing along the deep 

 ravines and valleys that run at right angles to the river at and 

 below Natchez ; and the same can, doubtless, be done at most 

 other points on the Mississippi River. But it must be done 

 by those who can be relied upon not to confound accidental 

 continuity with that which is essential. It is clear that here as 

 in the case of the Warrior or Tuscaloosa beds, a very critical 

 judgment must be exercised ; since one and the same gravel 

 may have been "worked over" several times during successive 

 geological epochs, and it must often be extremely difficult to 

 discriminate between that which is essential to the origin and 

 composition of the deposit, and that which is a local accident. 



Pending the determination of this point in the field, and 

 possibly with the aid of further microscopic research on the 

 samples already collected, I call attention to the fact that 

 intrinsically, the discussion of the "Tertiary" or "Quater- 

 nary " age of the Lafayette may easily be more a war of words 

 than of essentials. If preglacial, as is maintained- by McGee 

 and Chamberlin, it certainly stands at the very top of the 

 Tertiary series, if the beginning of the Glacial period be made 

 the criterion of the lower limit of the Quaternary. But this 

 is a purely arbitrary limit. It is quite certain that the condi- 

 tions under which, from any point of view, the Lafayette was 

 deposited, differed widely from those which presided over the 

 formation of all the other Tertiary stages in the Southwest. 

 During early Eocene times there prevailed in the upper por- 

 tion of the Embayment, estuarian, marsh and paludal condi- 

 tions, as is evidenced by the lignitiferous beds of that region. 

 Later we find steadily increasing marine conditions, represented 

 by the Claiborne, Jackson and Vicksburg terranes : at the end 

 of the latter epoch the Embayment had already lost so much 

 of its northward extension that the Gulf shore was only slightly 

 more convex than at present. Then comes the somewhat 

 enigmatical Grand Gulf terrane, clearly a brackish water 

 deposit, consisting largely of remarkably solid cla} r s and 

 (usually) very soft and more or less clayey sandstones. Of 

 recognizable fossils this formation contains only plants, so far 

 as has been definitely ascertained ; but certain calcareous beds 

 within it seem to contain the macerated, concretionary rem- 

 nants of a molluscan fauna, such as are now found over the 

 greater portion of the Port Hudson terrane, but which at some 

 points differentiate, into well defined fluvio-marine shells. 



