L. W. Milgard—Geological History of the Gulf of Mexico. 398 
} = equivalents, the Rotten limestone would be represented by his 
ort Pierre group. 
The distinctive features of these several groups become less 
marked the farther we advance northward, even in Mississippi. 
Non-fossiliferous or lignitic clays and sands mingle with the 
marine strata; and become altogether predominant, it would 
‘ seem, near the northern termination of the outcrop.* 
West of the Mississippi, the continuous Cretaceous outcrop 
does not extend as far northward as on the east side, by some 
150 miles. Nor have the more ancient lignitiferous beds 
Dias ilies ig 3 
SE. 
great rock-salt mass of Petite Anset, exhibit the 
teristics of the Ripley group; while deep borings have demon- 
strated the presence, for a thousand feet beneath, of the uniform 
Rotten limestone, such as it exists on the prairies of Mississippi 
and Alabama. I have elsewheret stated the stratigraphical as 
Well as lithological reasons which induce me to consider both 
the rock-salt of Petite Anse, and the sulphur and gypsum de- 
posits of Calcasieu, as lying within the limits of the Cretaceous 
formations. 
The data given by D. D. Owen seem to assign to the Cretace- 
ous strata of Arkansas a dip S. or slightly W. of S. The out- 
ers in Louisiana are too limited in extent for determinations 
of dip; but it can scarcely be doubted that they represent the 
Summits of a (more or less interrupted) ancient ridge, a kind of 
“backbone” to the State of Louisiana, whose resistence to 
udation has measurably influenced the nature and confor- 
_ Mation of subsequent deposits. It is fair to presume that from 
_ this ridge the strata dip toward the axis of the Mississippi 
_ Valley, to meet those on the opposite side; and the depth at 
Which these beds are found in the Calcasieu bores, seems to 
Indicate, on the western slope, a south-southwesterly dip of 
to four feet per mile. A glance at the map shows, never- 
_ theless, that the general form of the northern Gulf shore was 
_ Rot materially influenced by the existence of this axis of eleva- 
_ Hon, which probably was marked merely by a series of discon- 
_ Rected islands in the early Tertiary sea that, after the emergence 
* Fide Safford : 
p;t, Indicated on the map by the localities of Petite Anse, Chicot, Winfield and 
3 This Journal, Nov., 1869, p. 345. i : 
