274 EEPORT OP THE AMERICAN COMMITTEE. 
been found in tliese beds, Unios, etc., are fresh-water forms, and 
while pointing to the probability of Miocene or even later age, 
are still by no means conclusive. 
Concerning this formation Dr. Hilgard writes as follows : 
" As to the Grand Gulf series, the absence of any definite dip 
comparable to that of the Vicksburg beds, indicates an uncon- 
formity which I cannot exactly reconcile with the almost indefi- 
nite lithologlcal and stratigraphical transitions I have traced on 
the hillsides, falling off to northward into the Vicksburg prairies. 
I hold, as I have said in my article on the subject, that some- 
thing has happened at the end of the Vicksburg era that com- 
pletely changed the conditions of deposition, and may have 
covered a long geological time; perhaps both Miocene and Plio- 
cene, if Tuomey's S. C. Pliocene is relegated to the Post-Pliocene, 
as somebody has threatened to do. 
" You know that in the deep borings at New Orleans nothing 
that would indicate a Tertiary deposit was found. I can't help 
thinking that when the stratigraphy of the Vicksburg beds in 
Alabama and Florida has been studied, they will be found to dip 
much less than in Mississippi,* giving color to my conjecture 
that their dip at Jackson and Vicksburg is partly due to deposi- 
tion on a slope; if so, the apparent nonconformity between the 
Vicksburg and Grand Gulf may vanish. Loughridge's Barrens 
sandstone in Georgia seems to lie in precisely the same horizon as 
the Grand Gulf, and on the N.E. seems to connect with the South 
Carolina Miocene, equivalent to that, the Patapsco in Maryland. 
Perhaps the key to the situation may be found in Florida, in 
a section across the peninsula." 
And upon another occasion as follows: 
" I have no new light to give on the subject of the Grand Gulf 
beds. As a local [Gulf] feature, I consider them incapable of 
construction by themselves, with reference to the other Post- 
Eocene tertiaries. Besides, the mouth of the great Mississippi 
estuary is a bad place at which to study marine phenomena. 
Texas and Yucatan formations will doubtless be found to settle 
the question for good. I hardly think that the non-conformity 
* The explanation of the difficulties suggested by Dr, Hilgard may probably 
be found in the existence of undulations and faults in the Tertiary formation 
recently traced out in Alabama, and described in Bulletin !No. 43 of the 
U. S. Geological Survey. 
