84 



SCIENCE. 



[N. S. Vol. XVIII. No. 446. 



The difference of opinion, therefore, be- 

 tween Messrs. Smith and Aldrieh and myself 

 is in regard to names and their application 

 merely, and not a calling in question of the 

 accuracy of any observation made by them. 



It is an acknowledged fact, I believe, that 

 at least since the period of the Vicksburg 

 sedimentation, a considerable part of the 

 shores of the Mississippi embayment have 

 been and still to some extent are the seat of a 

 sedimentation of alluvial material in fresh or 

 brackish water containing fragments of vege- 

 table matter converted into lignite, and from 

 which only a few rare specimens of fresh- 

 water mollusoan fossils, turtle shells, etc., 

 have been obtained in half a century. The 

 rarity of fresh-water shells is proof that the 

 marshes or lagoons could not have been purely 

 fresh-water areas, the absence of oysters, etc., 

 shows that they were not permanently brackish, 

 and we are forced to offer the hypothesis that 

 fresh and salt water so alternated over the 

 area concerned, that inhabitants of neither 

 were able to maintain a footing and that the 

 organic remains found are either drifted from 

 elsewhere or the product of extremely local 

 and temporary conditions.- 



The earlier deposits of this kind, other 

 things being equal, we should expect to, and 

 I believe we do find at the greatest distance 

 from the sea and in the most consolidated 

 state; though a comparatively recent trans- 

 gression has carried unconsolidated sediments 

 over a large part if not all of the antecedent 

 deposits. Now it seems to me that in their 

 interesting communication Messrs. Smith and 

 Aldrieh have momentarily forgotten the his- 

 tory of research on this perplexing question. 

 Let us very briefly review it. 



The Grand Gulf sandstone, a rock ' superior 

 in hardness to granite itself ' was first named 

 by Wailes in 1854, who specifies as a typical 

 exposure that on the banks of the Mississippi, 

 at Grand Gulf, Claiborne Co., Miss., from 

 which the formation was named. Hence in 

 the allocation of names to portions of the sedi- 

 ments which have since been hastily included 

 under it, we must reserve for this particular 

 horizon the name of Grand Gulf. Wailes 

 believed that more tractable rocks to the east- 



ward were identical with this sandstone, but 

 everywhere it is described by him as a rock, 

 a hard or massive sandstone, suitable in its 

 softer phases for building stone, millstones, 

 etc. Beyond the Mississippi this sandstone 

 reappears in Louisiana, and according to Miss 

 Maury extends across the state and as far as 

 the Brazos Eiver in Texas. To the eastward 

 near Oak Grove, Florida, the typical sand- 

 stones according to Professor Harris and Miss 

 Maury ' pass beneath the (Oligocene) Oak 

 Grove sands, indicating that the sandstone is 

 approximately of the same age as the Chatta- 

 hoochee.''^ In Alabama the typical sandstone 

 is rare and the series corresponding ' usually 

 consists of clayey sands or joint clays ' ac- 

 cording to the same authority. 



In 1860 Hilgard, in his valuable report on 

 the ' Agriculture and Geology of Mississippi,' 

 considerably enlarged the scope of the forma- 

 tion, taking in clays, sands, consolidated and 

 unconsolidated, over a large area of country. 

 Later, as mentioned in my last communica- 

 tion, he canie to the conclusion that the series 

 included a succession of sediments of ages 

 between the Vicksburg and the drift. The 

 fact that at the typical locality the flinty 

 sandstone is succeeded by the unconsolidated 

 Lafayette or Orange Sand, is of course no 

 evidence of continuous sedimentation with- 

 out a break between the two, such as appears 

 to be the case in the aluminous clay of the 

 Chattahoochee, where no distinct line of de- 

 marcation is visible between the latter and the 

 so-called Lafayette conformably above it. 



As one of the problems to be solved this 

 state of things has long attracted the atten- 

 tion of the few geologists working on the 

 southern Tertiary. Some fifteen years ago I 

 received from Professor Smith what were 

 hailed as fossils at last from the Grand Gulf 

 sands of Roberts, Ala., a horizon which in 

 1894 Professor Smith included in the Grand 

 Gulf formation.f They were very imperfect 

 but fortunately contained one identifiable 

 characteristic Oligocene species of the Chipola 

 horizon. Subsequently Mr. L. C. Johnson 

 obtained from what he pronounced to be 



* Bull. Am. Pal., No. 15, p. 70, 1902. 

 t ' Coastal Plain of Alabama,' p. 102. 



