142 OIL FIELDS OF TEXAS-LOUISIANA COASTAL PLAIN, [bull. 212. 
if not stopped by some impervious barrier would continue until it 
reached the surface and then be dissipated. An essential condi- 
tion of any large accumulation is therefore an impervious stratum 
which shall check this upward course of the oil and restrain it in a 
porous reservoir rock below. Such impervious strata usually consist 
of fine clay, clay shale, or muddy limestone. If the bed is perfectly 
continuous, a few feet in thickness of clay or clay shale is sufficient 
to prevent any leakage from the underlying porous beds even under 
great pressure. In the Spindletop pool the cover is formed by a con- 
siderable thickness of alternating clay beds and limestones, the 
lowermost of which is the so-called "cap rock." 
ANTICLINAL STRUCTURE. 
When oil, whatever its origin, occurs in a porous bed along with 
water, it tends to rise toward the surface and continues to rise until 
it reaches the surface or meets some obstruction. If the obstruction 
is a perfectly horizontal stratum of impervious material, the progress 
of the oil is checked, but it does not accumulate in large bodies. If, 
on the other hand, the impervious stratum is inclined, the oil con- 
tinues to move upward along its under side until it meets a downward 
bend in the bed which it can not pass. The oil is thus trapped in the 
fold or anticline, and if the impervious bed is continuous over the 
crest it continues to accumulate and an oil pool is formed. The 
Appalachian field is characterized by low folds, which have a general 
northeast-southwest trend, parallel with the large folds of the Appa- 
lachian Mountains, and these folds have been of the highest impor- 
tance in the accumulation of oil in that field. In the Gulf Coastal 
Plain no structures are found which at all resemble the anticlines of 
the Appalachian field. The latter are undoubtedly due to horizontal 
compression of the earth's crust, but the Coastal Plain does not 
appear ever to have been subjected to such compression, and conse- 
quently the long regular parallel folds are wanting. The circular or 
elliptical domes which have been described as occurring at Spin- 
dletop, Damon Mound, and elsewhere are structures of a wholly 
different class, and could scarcely have been produced by horizontal 
compression. Although these domes are not strictly comparable with 
the anticlines of the Appalachian field, they are equally efficient in 
furnishing the structural conditions favorable for the accumulation 
of oil. 
SATURATION OF THE ROCKS AND CIRCULATION OF THE SATURATING 
FLUID. 
Sedimentary rocks below the immediate surface are generally satu- 
rated with water, either fresh or saline. In some cases, particularly 
in the Appalachian field, the rocks are dry, and under these condi- 
tions, even when sufficiently porous, they are not readily traversed 
