484 



SCIENCE. 



[N. S. Vol. XXIII. No. 587. 



mainly by Professor Dall, who has classed 

 them as Oligoeene and Miocene. 



Their thickness, as estimated by Messrs. 

 Pumpelly and Dall, is 200 feet, but the 

 well borings in Mobile have since shown 

 that a greater thickness must probably be 

 allowed. 



It will be remembered by those who have 

 kept pace with the progress of gulf coast 

 geology that up to 1881, when the last 

 paper of Hilgard's above referred to was 

 written, the Grand Gulf, a non-marine 

 formation, had always been observed to be, 

 along its northern border, in contact with 

 the Vieksburg limestone, and to occupy the 

 surface of the coastal plain (of Mississippi, 

 at least), thence southward to within ten 

 miles of the gulf shores. 



No other Tertiary formation younger 

 than the Vieksburg had then been observed 

 underlying it, or indeed outcropping at all 

 in this coastal plain, and none overlying 

 it older than the Stratified Drift or Orange 

 Sand (since called Lafayette), and its posi- 

 tion in the geological column seemed thus 

 to be definitely fixed, leading Hilgard to 

 the conclusion that 'its rocks alone repre- 

 sented on the northern border of the gulf, 

 the entire time and space intervening be- 

 tween the Vieksburg epoch of the Eocene 

 and the Stratified Drift. ' 



This view was generally accepted by the 

 geologists of the gulf coastal plain, since 

 similar relations between Vieksburg and 

 the Grand Gulf beds had been observed 

 and described at various points from Texas 

 to the Perdido River. 



At this time (1881) the Grand Gulf beds 

 had been identified in Alabama through 

 Washington and Baldwin counties to the 

 Perdido River, the boundary between Ala- 

 bama and western Florida ; and in Georgia 

 the siliceous strata of the wire-grass region, 

 since designated Altamaha grit, were cor- 

 related by Drs. Loughridge and Hilgard 

 with the Grand Gulf of Mississippi and 



Alabama;* but neither in Alabama, nor in 

 Florida, nor in Georgia, had the superficial 

 distribution of these beds been at all fully 

 mapped; only detached occurrences had 

 been noticed. West of the Mississippi 

 River, also, the Grand Gulf had been 

 identified and partially mapped through 

 Louisiana and Texas. 



The discovery by Langdon of the marine 

 Miocene beds of the Chattahoochee occupy- 

 ing a part at least of the position hitherto 

 thought to be monopolized by the Grand 

 Gulf, naturally necessitated some modifi- 

 cation of Hilgard's view, and this necessity 

 was emphasized by the subsequent discov- 

 eries mentioned of beds with marine and 

 estuarine shells of Miocene and Pliocene 

 age at other localities in Mississippi, Ala- 

 bama and Florida, in territory known to 

 be occupied, superficially at least, by un- 

 questioned and unmistakable Grand Gulf 

 strata. 



At first, as we have seen, the supposition 

 was put forward that the Pascagoula bed 

 was an estuarine deposit in the Grand Gulf 

 and a part of that formation, then, con- 

 sidering the conditions at Roberts in 

 Escambia County, Ala., where typical 

 Grand Gulf sandstones and clays (from 

 the relative positions of their surface out- 

 crops) appeared to lie between the Vieks- 

 burg limestone and the beds with the 

 Miocene shells, the Miocene age of the 

 Grand Gulf, and the fact of the gradual 

 replacement, coming eastward, of its non- 

 marine beds by marine equivalents, were 

 thought to have been established; and 

 finally, in the Chattahoochee River section, 

 it was thought that we had proof of the 

 complete replacement of non-marine by 

 marine strata. 



At that time the presence of Grand Gulf 

 beds of the characteristic non-marine type, 

 overlying, and neither interstratified with 

 nor replacing, the marine Miocene beds of 



* Tenth Census Report, Vol. V. 



