Between Calais and Valenciennes the sinking of mine shafts was surrounded by immense difficulties on account of the masses of subterranean water in great sheets at the level of the lowest valleys. Only the construction of tubbings, frameworks jointed like the stays of a barrel, could keep out the springs which flow in and isolate the shafts in the midst of the lakes, which with deep obscure waves beat against the walls. It had been necessary in sinking the Voreux to establish two tubbings: that of the upper level, in the shifting sands and white clays bordering the chalky stratum, and fissured in every part, swollen with water like a sponge; then that of the lower level, immediately above the coal stratum, in a yellow sand as fine as flour, flowing with liquid fluidity; it was here that th
