The term sill nowadays employed for a broadly concordant igneous intrusion is widely believed to have been derived from the local term for persistent hard beds in the Carboniferous sequence of NE England, in particular the Whin Sill. Despite the intrusive origin of the Whin having been demonstrated in Teesdale by Adam Sedgwick in 1827, for much of the nineteenth century the alternative extrusive hypothesis, of which John Phillips (1836) was the principal proponent, was widely favoured. There were three principal reasons why the intrusive origin was not more widely held, unquestioning acceptance of the erroneous belief of local miners that the Whin Sill was always at the same stratigraphical horizon, a perception that the Teesdale outcrops were not necessarily typical of the rest of the region and a reluctance to accept that the intrusion of such large volumes of magma over such a great area was physically possible. In the 1870s, first the work of Tate and then the detailed six-inch to one-mile mapping of the Geological Survey finally dispelled any notion that the sill was at a consistent stratigraphical level. Curiously, though the correct determination of the intrusive origin of the Whin Sill was one of Sedgwick's earliest and greatest achievements in Northern England, it was not deemed of sufficient importance to merit mention by his biographers.