MARYLAND GEOLOGICAL SURVEY 281 



more slowly. Moreover, as already shown, where cleared agricul- 

 tural land is long abandoned, this land quickly reverts to the original 

 forested condition. 



It is not to be presumed, however, that with a widespread and 

 complete removal of existing species a serious change would not be 

 made in the composition of the subsequent forest growth. The 

 reproduction of all trees, as with other plants, depends emphatically 

 on the presence of seed trees, be they far from or near the cleared 

 land. But the lumbering and other timber-consuming industries of 

 previous years have by chance left enough seed trees to assure the 

 perpetuation of all species for the present. 



The removal of large deposits of coal from beneath wooded areas 

 may permanently change the surface, kill the existing timber and 

 retard subsequent reproduction. 



As is well known to those familiar with these coal-mining opera- 

 tions, as much as possible of the big coal-veins, about 6 to 9 feet 

 thick and lying in a horizontal position, is taken out before aban- 

 doning the mines. In agricultural regions these worked-out coal- 

 beds may doubtless be left sufficiently shored up by pillars of coal to 

 prevent any collapsing of the surface. In the rougher hill country, 

 however, mostly with brush or forest cover, such expensive precau- 

 tions are not likely to be taken. The coal is taken out and the sur- 

 face left to sink or retain its position, as circumstances permit. When 

 coal-beds lie over one hundred feet below the surface, the unsup- 

 ported cavity seems not to result in any but an irregular depression 

 in the surface. Where the removal of deposits is nearer the surface 

 (30 to 75 feet), the final dropping of the surface is usually violent. 

 Cavernous pits are produced which engulf the forest growth in min- 

 gled masses of variously tilted, fallen and upright trunks; but much 

 of this continues to grow. The timber thus involved is almost en- 

 tirely inaccessible. These breaks in the surface are gradually much 

 smoothed in their more abrupt portions by the washing and sliding 

 of soil and rocks. Smaller vegetation covers the bare spots, and the 

 undermined rough surface is finally overgrown with brush and forest 

 trees. 



