430 PROCEEDINGS OF MONTREAL MEETING. 
is in the “‘ block” coal-field, where the coal-beds are thin, ranging from 3 to 5 feet 
in the centers of the basins to a few inches or nothing between basins. These 
beds are in this area sometimes found thickened up to 10 or 12 feet—that is, from 
three to four times their normal thickness. Figure 2 shows the detail, in part, of 
a thickened bed from Columbia No. 8 mine. In this case the coal-bed, normally 
3 feet 4 inches thick, has been compressed until i2 feet thick. As might be 
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Figure 2.—Sketch of detailed Structure of Part of a 3-foot 4-inch Coal Bed locally thickened to 12 feet. 
expected, the coal which is obtained at such points simply crumbles into slack 
when mined. At one point the strata in general have suffered until the two coal- 
beds worked in this area, which normally are 20 to 30 feet apart, are brought to 
within 6 inches of each other, the lower bed being at that point thickened up to 
43 feet and the upper bed from + to 6% feet. 
In a third direction the evidence comes from the peculiar joint structure com- 
mon in this area. As stated above, this area lies in the “‘ block’’ coal-field, where 
joint structure is developed in the greatest perfection. Figure 3 shows the normal 
joint structure of block coal. Figure 4 shows the way the same structure appears 
over most of this area. In some parts.of the mines of this disturbed district the 
Figure 3.—Diagram showing normal Structure of the upper Block Coal about Brazil, Indiana. 
normal block structure is developed, though with this slight difference, that the 
butt slips or joints are not quite perpendicular, but incline a little. More gener- 
ally, though, the face slips disappear and the butt slips become strongly inclined 
to the north, dipping as low as 55 degrees, as far as measured, and where the action 
has been still stronger a second set of slips are developed as shown in the right 
of figure 4; so that when mined the coal comes out having the shape of a huge 
wedge. 
