WILCOX FORMATION. 25 
and referred by Dr. W. H. Dall to the Midway group. Very little work has been done from 
Pontotoc south to Alabama. In that State the Midway is well developed and has been de- 
scribed in the Coastal Plain report already cited. 
Detailed work in Oktibbeha County by Prof. W. N. Logan, of the Agricultural and Me 
chanical College of Mississippi, failed to reveal any Midway beds. In Kemper County sonic 
time was spent during the present survey along the contact between the Cretaceous and 
Tertiary, but the Clayton limestone, if present, does not come to the surface. 
WILCOX FORMATION. 
This important division of the Tertiary occupies a large area in northern and central 
Mississippi. It was originally named the Lignitic by Hilgard, and Doctor SafTord, State 
geologist of Tennessee, termed it the Lagrange. A recent decision of the committee on 
nomenclature of the United States Geological Survey has substituted the name Wilcox, 
which is that of a locality in Alabama where the formation is typically exposed. The group 
is made up largely of highly-stratified siliceous sands, laminated clays of various colors, 
interstratified in places with beds of lignite, and lignitic clays. More or less glauconite or 
greensand is found throughout the group, and is usually associated with beds containing 
marine fossils. 
In Alabama, Smith has separated the old Lignitic group into six divisions. In each there 
are one or more marl beds, from which distinguishing fossils are obtained. He includes in the 
Lignitic the Sucarnochee and Naheola clays, which in Mississippi have been combined under 
the name Porters Creek and mapped with the Midway. 
The four remaining Alabama divisions of the Wilcox can be traced by the fossiliferous 
marl beds for a short distance into Mississippi, but they apparently fade out, and in the 
northern part of the State the Wilcox is barren of fossil fauna. It might be added that the 
fossils mentioned by Hilgard in his Geology and Agriculture of Mississippi, page 112, were 
later referred to the Middleton formation a of Tennessee, or Midway of the present report. 
There is no single lithologic stratum in the group which can be traced throughout the State 
except, perhaps, the 150 or 200 feet of chocolate-colored clays immediately underlying the 
Tallahatta buhrstone. This bed is fairly persistent across Alabama, and occurs at various 
places in Mississippi. There is also a great thickness of chocolate-colored clays in the deep 
well at Memphis, Tenn. Certain horizons in other parts of the group are more or less per- 
sistent in some localities. Lignite beds may be used as key strata in very limited areas. 
The white pottery and stoneware clays are well developed in northern Mississippi and repre- 
sent a fairly definite horizon for this section, but they gradually fade out in the central part 
of the State. 
Lignite is prevalent throughout the entire group, being found in numerous places in the 
lower, middle, and upper Wilcox. In places it is exceedingly pure, with a black glossy color 
very much resembling anthracite coal. When dry it burns with a bright-red flame. 
No accurate data are available for determining the thickness of the Wilcox. There are no 
deep wells known to have passed through it, so that the only means left for determining the 
thickness is by calculating the dip, principally from logs of wells which are supposed to 
obtain their supply of water from the same horizon in the Wilcox. Estimates of this kind 
have been made from the records of wells between Oxford and Batesville, between Bates- 
ville and Belen, and between Batesville and Riverside. The westward dip was thus found 
to be 16 feet between Oxford and Batesville, 17 feet between Batesville and Belen, and 18 
feet between Batesville and Riverside. 
The thickness, therefore, estimated from a westward dip of 17 feet per mile and a width of 
outcrop of 50 miles, is 850 feet. It is quite possible, however, that the dip flatten toward 
the west, since the arm of the Gulf at the close of Wilcox time was becoming narrower and 
the beds more horizontal. It is safe to assume, therefore, that 850 feet is the minimum 
thickness of the formation in northern Mississippi. L. C. Glenn reports that the deep well 
at Memphis passed through the Wilcox at a depth of 903 feet. 
a Bull. Qeol. Soc. America, vol. 3, pp. 511-512. 
