H KBNNEUY?j GEOLOGY OF EASTERN DIVISION. 21 
BASE OF COASTAL FORMATIONS. 
So far as the coastal formations are involved it is unnecessary for 
present purposes to go below the Fayette sands, as their outcrops 
form the northern limit of the Gulf Coastal Plain. In some localities 
it may not be necessary to go below the Frio clays. These clays in 
point of time are, so far as known, the last division of the Eocene 
in Texas. Owing to the overlapping of newer deposits and exten- 
sive erosion which has frequently completely cut through and removed 
portions of these Frio clays, great uncertainty exists in relation to 
this point, and it has been deemed best to assume the Fa} r ette sands 
as the base (or rather as lying immediately below the base) of the 
coastal formations. 
FAYETTE SANDS. 
The Fayette sands are sandstones and clays with occasional small 
irregular deposits of limestone carrying Lower Claiborne fossils, and 
sands containing imprints of leaves.- The limestones are not of 
frequent occurrence. 
The Fayette sands were considered Grand Gulf by Hilgard, Hop- 
kins, and Loughridge. Hilgard considered these and the next suc- 
ceeding clays to be the equivalent of his Mississippi beds. Penrose 
included them with the underlying Yegua claj^s and overlying Frio 
clays as the Fayette beds, and as such they are described in the early 
reports of the Texas geological survey. 
The structural difference between the underlying clays and the 
sandstone group led to a separation and the placing of the clays at 
the base of the Fa} 7 ette beds under the title of Yegua clays. The 
Frio clays were afterwards assigned to a separate division and the 
name Fayette was retained for the middle division of sandstones and 
clays. 
It is with these Fayette sands in their restricted definition that we 
have to deal. These beds form a well-marked horizon from the Mis- 
sissippi River westward through Louisiana and Texas past the Rio 
Grande into Mexico. 
On their northern side they show abrupt faces in numerous local- 
ities, indicating extensive erosion. When seen to the south they 
apparently slope off gently at a dip of only a few feet per mile. This 
dip is such that if the beds continued with uniform composition 
they should be found in the wells at Spindletop at depths not exceed- 
ing 1,200 to 1,500 feet. They do not occur in these wells, however, 
and it may be inferred that considerable erosion has taken place 
along their southern margin as well as on the northern side. That 
these sandstones formed the shore line for some time is evident, not 
only in the eastern division of the Coastal Plain, but also farther 
west, where the sandstones and the later deposits can be seen abut- 
ting against eacli other in such a manner as to indicate that the sands 
formed a steep shore line when the later deposits were laid down. 
