GEOLOGICAL HISTORY OF THE GULF OF MEXICO. 515 
matter here referred to is embraced in the author’s publications for 
ten years past. * 
The most ancient shoreline of this embayment is formed chiefly 
by the Carboniferous rocks. The oldest formation cropping out 
within these limits is the Cretaceous, which east of the Mississippi 
traverses the states of Alabama, Mississippi and Tennessee, 
reaching a short distance into Kentucky, along the Carboniferous 
ridge which borders the Tennessee river. The dip is 20-25° at 
right angles to the trend of the formation, which in its southerly 
portions is very distinctly subdivided into three chief groups, whose 
equivalents seem to be readily traceable in the Northwest. But 
towards the northern, narrow end of the outcrop, the distinctions 
become less defined, and the character more or less lignitic. 
West of the Mississippi, in Arkansas, the outcrop does not ex- 
tend so far north by some one hundred and fifty miles. It there 
corresponds to the middle group, the lowest not having thus far 
een recognized ; while the upper one appears to be represented 
in the series of Cretaceous outliers existing in Louisiana, forming, 
apparently, a Cretaceous ‘‘ backbone” to that state, whose south- 
ernmost point is, probably, the rock-salt mass of Petite Anse. 
The sulphur and gypsum of Calcasieu, likewise, seem to belong to 
this epoch, but their precise mode of formation can at present 
only be guessed at. 
During the Tertiary period, the shoreline receded from its ex- 
treme head in southern Illinois, to near the latitude of Baton 
Rouge, running nearly parallel to the present one. This seems 
to show comparative shallowness ; and this point is confirmed by 
the predominantly lignitic and lignito-gypseous character of the 
deposits, especially in the upper part of the embayment, where 
small outliers only of a marine character exist. The lignitie 
feature repeats itself throughout the predominantly marine de- 
posits of the later times; less so in Alabama than in Mississippi, 
while in Louisiana it largely predominates, owing probably to the 
marine 
_presence of the Cretaceous “backbone.” In general each 
group has its lignitic equivalents; and since lignitic outliers con- 
nect across Texas, with the great basins of the interior, a close 
— of their flora (and possibly fauna) may, it is hoped, enable 
p. on the Geology and Agriculture of pager oe bs Am. J. agin rra 
ma July 1866; Ibid. Nov. 1866; Ibid. Jan. 1867; Ibid. 859; Ibid. Nov. 1869. 
U. S. Engin. Dept. 1870; Am. J. Sci. Match, April and ‘ad: 
AMER. NATURALIST, VOL. V. 33 
