488 



SCIENCE. 



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



long ago emphasized, that, apart from local 

 steep dips in any direction, its strata could 

 nowhere be shown to have anything else 

 than approximately horizontal position, on 

 an average; and ignoring the still more 

 convincing circimistance that, with a thick- 

 ness of only 200 to 300 feet, it spreads over 

 an area in the state of Mississippi, of at 

 least 120 miles from north to south. If 

 it had the usual observed dips of the other 

 Tertiary formations in that state, the 

 Grand Gulf would lie between 3,000 and 

 4,000 feet below the surface at points near 

 the Gulf where it is certainly known to 

 be at the surface. 



MATERIALS. 

 The materials of the Grand Gulf are essentially 

 clays and sandstones, the latter generally alu- 

 minous and soft, and of white, gray and yellowish- 

 gray tints;' the sand being very sharp. Beds of 

 loose sand are unusual; but the clays are often- 

 times quite meager, though the sand contained in 

 them (as is the case in the sandstones) is usu- 

 ally quite fine. 



To this description by Hilgard it might 

 be added that the sandstones and the mas- 

 sive clays, which are often indurated into 

 a mudstone grading into a sandstone, are 

 frequently mottled with irregular red 

 splotches, in this particular, as well as in 

 others, resembling some of the material of 

 the Tuscaloosa formation of the Cretaceous. 



Stratified clays in thin layers are also 

 common in Alabama, and these neai'ly 

 always have a pink color. Pebbles of 

 small size, one fourth to one half inch in 

 diameter, are not uncommon in thin irreg- 

 ular bodies, generally at the base of sandy 

 beds. Much stress has been laid on the 

 occurrence of the sandstones and other in- 

 durated strata as being only in the presu- 

 mably lowermost (because most northerly) 

 beds of the formation ; but this is a mis- 

 take, since one of the most conspicuous ex- 

 amples of this occurrence is at P.ort Adams 



on the Mississippi River, in the southwest- 

 ern corner of Mississippi, at the Louisiana 

 line. In Alabama they occur down to the 

 Florida line, and in the Altamaha Grit re- 

 gion of Georgia— as shown by McCallie, 

 Harper, Burns and others— these indurated 

 beds, in every respect identical with those 

 of the type locality in Mississippi, have 

 been observed sporadically in eighteen 

 counties and in every part of this region, 

 down nearly to the Florida line in south- 

 western Georgia, and to within twenty-five 

 or thirty miles of the Atlantic coast on 

 Ocmulgee River. According to the e.sti- 

 mate above referred to of Dr. Harper, who 

 has devoted several years to the study of 

 this region, these rock outcrops do not 

 constitute more than one one hundredth of 

 one per cent, of the area of the formation. 



The marks on the map before you will 

 show this clearly enough, and ought to dis- 

 pose of the statement so often made that 

 the indurated parts of the Grand Gulf are 

 older than the rest— or that they represent 

 a different form from the less consolidated 

 parts. One might as reasonably hold that 

 the indurated crusts so often seen in the 

 Lafayette constitute a distinct formation 

 worthy of a distinct name. In general, 

 however, the statement of Hilgard holds 

 good, that 'there is a gradual increase in 

 clayeyness and a deci-ease of hardness, un- 

 til in the seaward portions of the forma- 

 tion we find chiefly stiff blue or green and 

 more or less massy clays, '" and I might 

 add for Alabama, laminated clays and 

 coarse sands. 



Lignite and gypsum are characteristic 

 of the materials both in Alabama and in 

 Mississippi, and in both states the wood is 

 often silicified. The lignitie faeies is, how- 

 ever, more common in Mississippi. The 

 lignitie mattei's generally occur in rather 

 limited lenses in the other strata. Some 



' Am,. Jour. Sci., Vol. XXII., p. 59. 



