72 OIL FIELDS OF TEXAS-LOUISIANA COASTAL PLAIN, [bitll.212. 
feet of oil rock and a distance of 100 feet into a white limestone under- 
neath. No samples of the latter were obtained, but from the descrip- 
tion given by the driller it is regarded as at least probable that this 
bed is gypsum rather than limestone. The thickness of 50 feet for the 
oil rock indicates that the bed thins out toward the western edge of 
the pool. 
The Higgins No. 3 well passed through 19 feet of oil rock at a depth 
of 855 feet. This was cased off and the drilling continued, with a 
view of determining whether or not there were any lower oil-bearing 
horizons. A second bed of oil rock was reported, 21 feet in thickness, 
below which the drill penetrated 70 feet of rock similar to that found 
in the bottom of the Robertson well. The Higgins Company put 
down another test well in the immediate vicinity of the No. 3, which, 
after passing through the oil rock, entered gypsum at about 1,040 feet 
in depth, and at 1,650 feet encountered salt, which continued to a 
depth of 1,960 feet, where the drilling stopped. The entire thickness 
of the salt is not known. 
STRUCTURE OF SPINDLETOP POOL. 
In the records of wells drilled on Spindletop the only horizon which 
can be identified with any degree of certainty is the oil rock itself. 
It is generally supposed that the so-called cap rock on which the casing 
is set represents the same stratum in every case, but this is by no 
means certain. Each driller decides when he has reached a bed suf- 
ficiently solid to set his casing, and this point probably varies rather 
widel} 7 , which explains in some measure the differences in reported 
depth to the cap rock in closely adjoining wells, and some, at least, 
of the apparent irregularities in the surface of the oil rock. Elimi- 
nating these minor irregularities the structure of the pool appears to 
be that of a dome or quaquaversal with steep sides and rather flat 
summit. The dome in the beds at the oil-bearing horizon coincides 
closely with the surface elevation, Spindletop Hill, although the dips 
of the beds around the margin of the dome are much steeper than the 
corresponding slopes of the land surface. 
The accompanying sections across the pool on the lines shown on 
the map, nearly at right angles to one another, show the structure so 
far as it has been made out. Outside of the producing territory the 
numerous wells which have been put down, some to depths of more 
than 2,000 feet, fail to reach any stratum which can definitely be 
correlated with the "cap rock." Hence the lines indicating structure 
can be extended beyond the limits of the field only by inference. 
All the indications are, however, that the dip away from the dome is 
very abrupt, possibly more so than shown on the sections. These 
sections are drawn upon a natural scale — that is, the horizontal and 
vertical scales are the same — and the lines represent the actual slopes 
of the beds so far as they can be determined from data now in hand. 
