58 EW. Hilgard—Later Tertiary of the Gulf of Mexico. 
Art. XII.—The Later Tertiary of the Gulf of Mexico; by K. 
W. Hinearp, Berkeley, Cal. With a map (Pilate ITI). 
In view of the late earen of the Coast Survey chart 
of soundings in the of Mexico, and of the observations 
of Dr. Eugene A. Sorith’o on the Geological Formations of Florida 
(this varie April, 1881), I desire to summarize briefly the 
acts upon ich my hypothesis of a temporary and partial 
isolation of NG Gulf ea the Atlantic Ocean during the later 
portion of the Tertiary period, is based. shall add thereto 
some additional facts that have since been brought to my 
knowledge, Radar the more remote portions of the group 
of deposits to which, from its most .accessible and representa- 
tive sepoecee at the town of Grand Gulf, on ts sara 
aba I have given the name of “ Grand Gulf | 
posits , on the entire mainland border of the Gulf of Mantes 
from Florida to the Rio Grande. In the portions lying near 
the main axis of the Sk trough, the uppermost strata 
of the or ae rocks show, by the constant intercalation of 
with common and Glauber’s 5 atte The same is true of the 
lower portions especially, of the overlying Grand Gulf rocks; 
so that throughout the regiou occupied by the latter, few well- 
waters obtained peices them are fit for daily use, ‘and many 
are strongly mineral. 
At their lines caf contact, the Vicksburg and Grand Gulf 
rocks consist aia throughout of lignito-gypseous, laminated 
ole Ys passing u into more sandy materials: they are not 
nsibly uncon pethabls in place; but while the Vicksburg 
rooks show at all long exposures a distinct southward dip of 
some three to five degrees, the position of the Grand Gulf 
strata can rarely be shown to be otherwise than nearly or quite 
horizontal on the average; although in many cases faults of 
subsidences have caused them to dip, sometimes quite steeply, 
in almost any direction. They, however, lie high on the hill- 
tops between the towns of Vicksburg and Grand Gulf, and 
disappear sg the water’s edge near the Louisiana line, under — 
the gravel beds of ane Stratified Drift. 
