Regarding mines on mountains and mines on flat land:
It all depends on the coal seem. Most seems are horizontal, unless the rock structure it is in was shifted into mountain form, in which case, the seems are not so horizontal. In Appalachia, for example, coal seems could and did start on the side of a mountain, and slope down. In North America, coal seems are relatively thick, compared to in England, where the average seem was only 2' thick.