8 



depth as the thickness of the underlying coal vein is the result. 

 When the seam is deeper underground the effect may be merely a 

 crack with a slight settling. It is reported that this cracking of the 

 surface has taken place when the coal lay at a depth of 300 to 500 

 feet. In localities where the coal lies in general 50 to 100 feet below 

 the surface the lower margin of a hill may have a belt of sink holes 

 due to mining and the upper portion of the hill remain unbroken, 

 though occasionally cracked. 



Breaking or cracking of the surface lowers the permanent water 

 level to the mine. Local wells and springs run dry, and small 

 streams sometimes disappear. On such situations older trees die as 

 a result of the change in drainage conditions, but young sprouts and 

 seedlings adapt themselves to the new conditions. Heavy soil that 

 was formerly poorly drained has sometimes been improved by the 

 increased drainage in the same way that tile draining improves 

 similar land elsewhere. Areas where there are many sink holes are, 

 however, comparatively valueless for agriculture. 



EFFECT OF COKE PRODUCTION. 



A large amount of coal mined in western Pennsylvania is used at 

 the mines to make coke to be shipped for use at iron and steel mills. 

 The manufacture of this product destroys the existing timber and 

 prevents forest planting in the immediate vicinity of the works, 

 because the fumes from the coke ovens contain sulphur gas, which 

 causes the death of all vegetation subjected to a continuous draft of 

 this smoke. The action of the sulphur gas on the leaves is corrosive. 



Much land is protected on account of the hilly character of this 

 region. If the ovens are situated at the base of a hill against which 

 the prevailing wind carries the smoke, the hillside soon becomes 

 bare of vegetation, but on the opposite side of the hill crops may be 

 grown nearly to the top. The injurious effect of the smoke is notice- 

 able, however, at a greater distance in those situations where the 

 wind carries the smoke up a small, narrow valley. Crops several 

 miles away are often blackened, but do not seem to be injured. 

 The death of orchards and forest trees, caused by the changed con- 

 ditions of drainage already described, is frequently attributed to 

 the action of coke smoke. 



In some instances by-product ovens have been established which 

 collect and condense the sulphur gas to make commercial sulphuric 

 acid. 



OPPORTUNITY FOR FOREST PLANTING. 



The occasion for forest planting rests primarily upon the growing 

 need of pit props, and is intimately related to the whole industrial 

 development of southwestern Pennsylvania. 



