The prevailing structure of the Appalachians consists of elongate longitudinal folds that trend northeastward, most of which are over-turned toward the northwest and many of which are broken and over-thrust in that direction. These folds were produced by the yielding of the rocks to earth forces that compressed them at right angles to the axes of folding during Paleozoic time, largely at or near its close. In the Piedmont portion of the Appalachians another form of structure occurs, block-faulting, which was produced by the settling and adjusting of great earth blocks in Triassic time, after the compressive forces sub‐sided or were relieved. Triassic sediments, which had been deposited in basins produced by the sinking of great longitudinal areas, were further depressed at or near the close of the Triassic period by normal drop-faulting on one side of the basin and the tilting of the depressed block. Although . . .