E. W. Hilgard—Geology of Lower Louisiana. 85 
very extensive horizontally, or that such masses should occur 
frequently in the coast region. 
A mass of salt 144 acres in extent and 38 feet thick is, how- 
ever, a handsome specimen, even if these dimensions should 
represent maxima. The great difficulty in mining it, hereto- 
fore, has been the influx of water through the gravelly strata 
overlying. But it has most probably been attacked, thus far, 
at its lowest surface level. Wherever elsewhere the Orange 
Sand formation prevails, it rests on a deeply denuded surface ; 
and “hills within hills” are o common occurrence. 
From the data thus far obtained it appears that the same is 
the case with the rock-salt mass, and that its surface roughly 
conforms to the hills and valleys now existing. orkin 
should be begun at higher levels ; and it would not surprise 
me to learn that the auger had shown the mass’to be accessible 
by level adits in lieu of shafts, on the-hillsides. The interior 
of the solid mass once gained from a point secure from surface 
water, all difficulty would be at an end. 
Geological History of the Lower Mississippi Valley. 
It appears from the facts stated in the preceding pages, that 
after the termination of the epoch of that Eocene period, repre- 
sented by the Vicksburg group of fossils, down to the Quater- 
nary era, marine deposits ceased to be formed on the northern 
border of the basin now represented by the Gulf of Mexico. 
I have acquired the certainty of the existence, over a large 
portion of northern Louisiana, of the “Grand Gulf” series of 
rocks, From specimens in the collection of the New Orleans 
Academy of Sciences, it appears that apart from the usual 
materials forming these beds in Mississippi, they assume in the 
Harrisonburg region the character of compact limestone, which 
in places is said to be fossiliferous, and would thus furnish the 
clue to the age of the Grand Gulf group, for which I have 
vainly sought in Mississippi. The problem is one of great 
nterest, as it involves the question whether or not the Mexican 
gulf has, within comparatively modern times, been disconnected 
from the Atlantic ocean. The absence of the cauldron in which 
the Gulf Stream is concocted might have exerted climatic 
influences reaching beyond the American continent, and would 
explain many discrepancies between ancient and modern faunas 
on the shores of the Atlantic. oe ee 
It appears that similar limestones, almost assuming the 
character of black marble, occur in Landry parish, near 
Opelousas, Whether the southern outline of the formation — 
aang thence toward the Calcasieu region, where petroleun has 
been found, or whether it trends northwestward into the par-— 
i 
