70 OIL FIELDS OF TEXAS-LOUISIANA COASTAL PLAIN, [bui.l.212. 
From their size and position it is, however, more probable that these 
so-called bowlders are fractured beds brought into their present con- 
dition by the uplift of the dome. Following these limestones is 
another series of clays, sands, shells, and limestones extending down 
to the so-called cap rock, which is a limestone of the same age as the 
overlying beds. It is of variable thickness, ranging from 3 to 50 feet. 
Throughout the beds, from the depth of 500 feet downward to the cap 
rock, occasional oil-bearing horizons are met with. Some of these 
exhibit considerable strength, and have in several wells been heav}^ 
enough to interfere with the drilling. Very heavy gas pressures have 
also been met with between the 500-foot sandstone and the cap rock. 
The depths at which the cap rock has been struck varies between 
900 and 1,000 feet. A few have fallen below the 000-foot mark, while 
some have slightly exceeded 1,000 feet. 
CHARACTER OF THE OIL ROCK. 
The oil rock is a dolomite, fairly uniform in composition and 
structure.' It is not a pure dolomite, but contains a considerable 
preponderance of calcium over magnesium carbonate, sufficient to 
cause it to effervesce rather briskly in cold dilute hydrochloric acid. 
The most striking characteristic of the oil rock is its extreme 
porosit}^. In some wells, under the pressure of the escaping oil and 
gas the rock was broken down and large quantities were carried up the 
casing to the surface, appearing as coarse sand or gravel, the frag- 
ments varying from a very small fraction of an inch to an inch in 
diameter. In addition to this disintegrated rock many large frag- 
ments were thrown out by the gushing oil, their size being limited 
only by the size of the casing. These fragments always have a porous 
structure, and even their most compact portions, as shown under the 
microscope, contain a larger proportion of vacant space than most of 
the oil-bearing Trenton dolomite of Ohio and Indiana. In addition 
to these minute spaces between the crystals composing the rock, such 
as characterize ordinary oil sands, the rock contains many large 
cavities, certainly as much as an inch across and probably very much 
more. Naturally the maximum size of these cavities can not be 
determined from the rock fragments which form their walls, but 
there appears to be nothing unreasonable in the supposition that 
they may in some cases be measured in feet rather than in inches. 
Reports from drillers of tools dropping several feet after passing 
through the cap rock are in support of this view, though such reports 
are always to be accepted with caution. 
The cavities in the oil rock are always lined with a layer of crystal- 
line calcite, the free ends of the crystals extending into the open 
spaces. It is evident, therefore, that the latest action of the circu- 
lating liquid in these cavities has been that of deposition rather than of 
solution, although the latter must have been at one period extremely 
