SUBSIDENCE
Not the least important application of geology to slides is in relation to deep mining operations. While the mining geologist has been principally engaged in exploration and development of ores, he is now beginning to be called in to interpret the great earth movements caused by the sinking of the ground over mining openings. For instance, the long-wall method of coal mining has resulted in a slow progressive subsidence of the overlying rock, affecting overlying mineral beds and surface structures over great areas. Detailed studies have been made of this movement, in order to ascertain its relation to the strength and structure of the rocks, its relation to the nature of the excavation, its speed of transmission, and the possible methods of prevention. German scientists have perhaps gone further with this kind of study than anyone else. In an elaborate investigation of subsidence over a coal mine in Illinois,[67] unusually complete data were obtained as to the nature, direction, and speed of the transmission of strains through large rock masses, and as to their effect in producing secondary rock structures.