398 E. Wj HUgard — Age and Origin 



While the Grand Gulf beds thus indicate a predominance of 

 fresh-water conditions for which the end of the preceding 

 (Yicksburg) epoch has not prepared ns, jet it follows the older 

 Tertiaries in that it offers no evidence of any other than quiet 

 modes of shore and marsh sedimentation, undisturbed, so far 

 as we have seen, by any invasion of fluvial conditions. Cross- 

 bedding even is unusual, and what there is is clearly of the 

 kind induced by wave action ; gravel is almost foreign to the 

 formation, although sometimes seen near the axis of the Em- 

 bayment. At the end of the Grand Gulf epoch the Embay- 

 ment had practically ceased to exist ; the northern Gulf-shore 

 line was almost exactly parallel to the present one, and all this 

 had been done in a quiet, systematic, orderly way, during a 

 slow upward movement of the land. 



With the next succeeding formation, the Lafayette, there is 

 a total change both in the material and its mode of deposition, 

 preceded by deep erosion of the surface of the older forma- 

 tions. So far from continuing the advance of the shore-line 

 into the waters of the Gulf of Mexico, we suddenly have 

 (according to McGee's views) the latter advancing at least to 

 the head of the Mississippi Embayment, and according to 

 Chamberlin, clear up to Keokuk, by a general depression. 

 But this general depression does not carry marine features 

 backward over the land ; on the contrary, while not a marine 

 fossil, even in the most " demoralized '' condition, can be 

 found in the entire enormous area covered by the formation, 

 we have a marked increase in all features that we habitually 

 look for in a fresh-water, fluvial formation ; including the evi- 

 dences of violent currents, with a total absence of beach lines 

 to show either the advance or recession of the supposed sub- 

 mergence. The only evidence that might be thus construed 

 are the gravel beds; but as these run in the main parallel to 

 the axis of the Embayment, while according to the Tertiary 

 regime they should run crosswise / and since in the very axis 

 of the Embayment the material of these gravels gives evidence 

 of transportation from north to south, on a slope materially 

 greater even than exists at the present time : it seems to me 

 very difficult to construe the gravel beds into beach shingle. 



It seems to me, further, that such an extraordinary conti- 

 nental oscillation as that which brought about the change from 

 the quiet Grand Gulf epoch to the stormy one of the Lafay- 

 ette, would in most cases in itself constitute good cause for 

 drawing a taxonomic line, as such lines are usually drawn. It 

 can hardly be doubted that the change from the Grand Gulf 

 to the Lafayette regime must have resulted in a material 

 change in the character of the aqueous fauna, if any there was. 

 We may therefore well question whether such events may not 



