hayes 1 OIL FIELDS OF TEXAS-LOUISIANA COASTAL PLAIN. 349 
pool. It occupies an oval area about 3,000 feet in length and 2,700 
in width, containing approximately 200 acres. The depth to the sur- 
face of the oil rock varies between 000 and 1,000 feet, a few wells only 
being outside of these limits. The oil rock is a crystalline dolomitic 
limestone, having an extremely porous structure. The most compact 
portions of the rock, as shown by the microscope, contain a larger 
proportion of vacant space than most of the oil-bearing Trenton dolo- 
mite of Ohio and Indiana. In addition to these minute spaces 
between the crystals of the rock, such as characterize ordinary oil 
sands, it contains many large cavities, certainly as much as an inch 
across, and probably much more. While no accurate determination 
of the relative volume of the open cavities can be made, it can hardly 
be less than one-third, and may be somewhat more when account is 
taken of the minute spaces between the crystalline grains. The 
exceptional character of this oil rock explains in a measure the 
remarkable features of the Spindletop pool. Its extreme porosity 
favors the storage of a very large volume of oil, and also favors the 
yielding of this oil with great rapidity when the reservoir is tapped. 
It also favors the early exhaustion of the oil in the pool and its rapid 
replacement by the underlying brine. It should have suggested to 
those concerned in the development of the pool that a few wells prop- 
erly distributed would have drained the pool quite as effectively as 
the large number which have been drilled. 
Associated with the dolomite, which forms the oil rock, is consider- 
able selenite or crystalline gypsum. Another abundant accessory min- 
eral is native sulphur. Large crystals, an inch or more in diameter, 
have been obtained from many of the wells, and it is reported by sev- 
eral of the drillers that the oil rock is overlain by a bed of sulphur, in 
some cases reaching a thickness of 40 feet. The thickness of the oil 
rock throughout the greater part of the pool is not known, though it 
has been penetrated to a depth of 96 feet. Toward the western edge 
it is probably less than 50 feet thick and, as shown by the Robertson 
well and the Higgins No. 3, is underlain hy about 100 feet of white 
gypsum. Below the gypsum the Higgins No. 3 penetrated rock salt 
to a depth of 310 feet without passing through it. 
Associated with the oil is always found a large amount of gas, and 
at several localities this form of hydrocarbon is found unaccompanied 
with oil. The composition of the gas has not been carefully investi- 
gated, but it is known to contain in addition to the light hydrocar- 
bons a large proportion of sulphureted hydrogen. 
At nearly all points in this field where oil has been found in com- 
mercial quantities it occurs under great pressure, which gives rise to 
the familiar phenomenon of gushing. Just how high the pressure 
has been in the Spindletop pool is not known, but it appears to vary 
between wide limits. In some wells it has shown almost explosive 
violence, blowing out casing and breaking heavy cast-iron valves. 
