From the northern boundary line of western Pennsylvania to cen- 

 tral West Virginia the regularity of the dip of the rocks is modified 

 by low, broad folds, which become pronounced ridges in the south- 

 eastern portion and are parallel to the general Appalachian folding 

 to the east; but the general topography is that of a plateau descend- 

 ing gently toward the west, more or less dissected by valleys, accord- 

 ing to the degree of erosion. The region is drained by the Ohio and 

 its tributaries, of which the most important are the Allegheny and 

 the Monongahela rivers. 



The coal-bearing formations of the northern Appalachian coal 

 field consist principally of shales, sandstones, and conglomerates, 

 with occasional beds of limestone, fire clay, and coal. The chief 

 source of coal in Pennsylvania has been the Monongahela and Alle- 

 gheny formations. Statistics compiled in 1902 by the United States 

 Geological Survey report Pennsylvania as having one-half the coal 

 and two-thirds of the coke production of the United States. 



The room and pillar system is in use in nearly all mines through- 

 out the region. The coal is either hand or machine mined, and is 

 removed from the mines in cars by electric, compressed-air, cable, 

 or steam haulage. Horses or mules are used in the mines to convey 

 the cars to the main haulages. Wood is required for the following 

 principal uses: 



1. Large and small material in buildings, tipples, etc., above 

 ground. 



2. Mine props or posts in the mines. By law the minimum size 

 at the top end must be 16 square inches. The length varies from 

 6 to 10 feet, according to the thickness of the seam being worked. 

 The more durable species, such as oak, chestnut, and locust, are 

 used for roof support in the main haulages, but any other species of 

 wood can be used elsewhere. 



3. Ties for mine railways. The ties in common use are 5 feet in 

 length, with a cross section from 4 to 6 inches in width and depth. 

 Durable woods are used under the tracks of the main haulage ways, 

 but almost any species on temporary branches. 



4. Large-sized timbers in the mine. These require durability and 

 strength, and are used at the mine entrance, or in special cases for 

 roof support. 



Mining frequently affects the physical character of the surface of 

 the land. With the final removal of the coal between the rooms 

 the roof of the mine tends to fall. This is an advantage in mining, 

 since it' relieves the pressure from above on the adjacent unmined 

 portions of the seam. By withdrawing the pit posts into a line 

 next to the unmined portion the fall of the rock above the mined 

 portion can usually be effected. Wherever the coal lies close to the 

 surface the fall-in extends to the surface and a sink hole of the same 



