COMPRESSED STRUCTURE IN WESTERN INDIANA. 431 
In the second place, as indicated in figure 3, the joints are normally confined to 
the coal as though their cause lay in the coal itself; and even when these joints 
are open, as they often are, to the extent of several inches or a foot, the roof re- 
mains smooth and unbroken. But in this disturbed area all the strata suffer alike, 
and, as indicated in figure 4, these oblique joints penetrate the roof and floor. 
From their generally extending up to the surface and so forming convenient pas- 
sage for water, they are commonly known locally as ‘“‘ water slips.” Where both 
sets exist they render the roof very unsafe, as they cut it up into wedge-shaped 
blocks that tend to come down readily. 
In the third place, the faces of the slips normally, though often quite even, are 
never polished, while in this area they are commonly slickened and highly pol- 
ished—a result usually observed where compressive action has acted on rocks. 
——————— =e 
—— ——oe 
> —————— 
North 
Bench/Minti ng x 
S LT 
= ——S LSS SSS SS 
Se BSS Sas a 
SS ———LFQF SSS (~ =~ 
BI FEF Bf Fe Slay. 
Sc = ea SOS 
Ficure 4.—Joint Structure as modified by tangential Pressure. 
Section along north and south entry in Columbia No. 4 mine. 
In places these inclined joints, instead of running regularly, are curved and of 
such shape that the coal bears a striking resemblance to cone in cone structure on 
a very large scale, the surface of the cones being highly polished. 
Over the less disturbed portion of the block coal area the coal basins still lie so regu- 
larly that the lowest points of the old basins are still the lowest points of the coal 
beds and the points where the coal is the thickest, while the original rims of the 
basins still rise 10 to 20 feet above these basins and are the areas of thin coal. But 
in this district no such rule holds, the coal as often getting thick to the rise as thin, 
showing that the topographic irregularities of the beds here are due to subsequent 
earth movements, and thus bear indirect testimony in line with the evidence 
already given. 
In conclusion, it may be stated that the evidence indicates that the pressure 
acted in a north-and-south line, or with the strike. JI am not as yet prepared to 
offer any suggestions as to cause of this pressure. 
In the absence of the author the following paper was presented in 
abstract by T. C. Chamberlin : 
NIAGARA GORGE AND SAINT DAVIDS CHANNEL 
BY WARREN UPHAM 
This paper is printed as pages 101-110 of this volume. 
LXIJI—Butu. Gror. Soc. Am., Vou. 9, 1897 
