HAYKs.j OIL FIELDS OF TEXAS-LOUISIANA COASTAL PLAIN. 347 
by a greater distance than a few hundred feet. Even in the Beaumont 
district, where so much drilling lias been done, it is possible to make 
only general statements regarding the stratigraphy. A part of this 
uncertainty results from the difficulty of obtaining an accurate record 
by means of the universally employed rotary drill, but a part also is 
due to the extreme variabilitj 7 in the character of the beds. 
STRUCTURE. 
The beds making up the Coastal Plain formations were deposited 
near the margin of a sea which varied in depth from time to time, or 
upon a coastal belt as wave-built beaches on river flood-plain deposits. 
The surface on which they were laid down had a gentle slope toward 
the southeast, and this slope was increased during their deposition 
by a slight tilting. Hence the beds all have a gentle dip to the south- 
east, but the lower or older beds have a somewhat greater dip than 
the higher or newer ones. 
While this gentle southeast dip is the prevailing structure through- 
out the Coastal Plain, it is interrupted at numerous points by low 
oval domes in which the beds dip away from the center in all direc- 
tions. This structure has been found to characterize all hills or swells 
which interrupt the even surface of the Coastal Plain. These domes 
do not appear to have been formed by lateral compression, such as 
has given rise to the anticlines of the Appalachian field, but rather 
are due to some force acting vertically and lifting a small portion of 
the earth's crust. This force appears to have become active some time 
during the Tertiary and to have continued since the deposition of the 
recent Beaumont clays. 
CONDITIONS FOR THE ACCUMULATION OF OIL. 
The conditions which are essential for the accumulation of oil and 
gas in commercial quantities are everywhere the same. They are 
(1) a source for the oil, either organic or inorganic; (2) a porous 
stratum which may serve as a reservoir; and (3) an impervious cap 
rock which will prevent its escape. Conditions which favor its accu- 
mulation, but are not always essential are (4) gentle undulations of 
the strata forming anticlinal arches or domes; and (5) complete satu- 
ration of the rocks with water and its slow circulation. 
In the Gulf Coastal Plain there appears to be a very large amount 
of oil disseminated through the several thousand feet of undertying 
Tertiary and Cretaceous, and possibly also Carboniferous strata. 
Scarcely a well has been drilled in this region to any considerable 
depth which has not encountered traces of oil in some of the beds 
passed through. There is also an abundance of porous beds adapted 
to form reservoirs for the oil. These are unconsolidated sands and 
gravels, and in some cases, as at Spindletop, a very porous limestone 
or dolomite. The impervious cover required to retain the oil and pre- 
