490 



SCIENCE. 



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



Gulf. This, with the 150 feet of the hill, 

 would make the thiclmess something over 

 300 feet. 



From its structure and distribution it is 

 difficult to avoid the conclusion that the 

 Grand Gulf is a sort of blanket or mantle 

 formation spread over part of the Vicks- 

 burg; in places over parts of older forma- 

 tions; and over all the Miocene and later 

 Tertiaries, with practically no general 

 southward dip more rapid than the descent 

 of the general land surface. In this it 

 resembles the next overlying formation of 

 our coastal plain, viz., the Lafayette, or 

 Orange Sand of Dr. Hilgard. The latter, 

 however, overlies a far greater number of 

 formations, including the entire coastal 

 plain series and even part of the Paleozoics 

 and Crystalline schists. The Lafayette is 

 also composed of siliceous materials, but 

 my experience in Alabama is that the clays 

 are comparatively rare. The prevailing 

 material is a red sandy loam with beds of 

 rounded, water-worn pebbles in irregular 

 bodies at the base. Very often the red 

 loam passes into a sandier phase of lighter 

 color— generally yellowish— before the peb- 

 bles are reached. The thickness can rarely 

 be shown to be more than twenty or twenty- 

 five feet at any one place, unless the ma- 

 terials are filling erosion hollows in the 

 underlying formations. A characteristic 

 feature in Alabama is the almost total ab- 

 sence of evenly stratified beds of any kind ; 

 the red loam at the surface seldom shows 

 any lines of stratification; the sands and 

 pebbles almost invariably exhibit cross- 

 stratification or false-bedding, due to depo- 

 sition from swiftly flowing currents; lami- 

 nated clays I have not seen at all in this 

 formation in Alabama. In this we have a 

 very marked distinction of the Lafayette 

 from either the Tuscaloosa or the Grand 

 Gulf, with both of which it has some fea- 

 tures in common. 



GENESIS. 



To account for many of the phenomena 

 of the Grand Gulf formation Dr. Hilgard 

 has from the first insisted that foremost 

 among the conditions of its accumulation 

 was exclusion of the sea,' or at least such 

 obstruction of the communication across 

 the still submerged peninsula of Florida as 

 to render the influx from the interior of 

 the continent predominant over the orig- 

 inal supply of sea-water. Later, in 1881, 

 after soundings" in the Gulf of Mexico had 

 revealed the topography of the gulf bottom, 

 and the existence of a submerged shelf of 

 100 to 130 miles width along the coast, he 

 suggested that even an elevation of 450 

 feet (which seems to be proven for the 

 Mississippi embayment) would convert 

 the whole gulf border into a region of 

 shallows out to the 100-fathom line, in 

 which the waters would be kept perma- 

 nently freshened by the continental drain- 

 age. It may be remarked in this connec- 

 tion that such an elevation would also raise 

 a portion of the present sea bottom into 

 dry land, and would not help to explain 

 the accumulation of Grand Gulf strata far 

 inland of the present coast, unless we as- 

 sume that this uplifting of the submerged 

 shelf was accompanied by a downward 

 warping of a belt farther inland. Nor 

 would it help in the explanation of the 

 occurrence of this formation on the At- 

 lantic side of the Georgia watershed, and 

 through South Carolina. 



In the Tuscaloosa or Potomac formation 

 of the Cretaceous we have an almost par- 

 allel case. In their component materials 

 and in their mode of accumulation the two 

 formations show striking similarities; in 

 both are only vegetable remains, or those 

 of land and fresh-water origin; both must 

 have been accumulated in sounds, partially, 

 at least, cut off from the sea, and the diffi- 

 culties in suggesting the precise nature and 



' Am. Jour. Sci., Vol. 11., December, 1871. 



