Along its main outcrop, the sill dips gently eastwards below the portion of the Carboniferous Limestone series which overlies it. But so slight are the inclinations, so gentle the undulations of the rocks in this part of the country, that far to the east of that outcrop the sill has been laid bare by the streams which in the larger dales have cut their way through the overlying cake of Carboniferous strata down to the Silurian platform on which they rest (Fig. 176). Among these inland revelations of the eastward continuation of the sill under Carboniferous Limestone strata, the most striking and best known are those which have been made by the River Tees, and of which the famous waterfalls of the High Force and Cauldron Snout are the most picturesque features. The distance of the remotest of these denuded outcrops or "inliers" from the main escarpment is not less than 20 miles. It is not possible to form an accurate estimate of the total underground area of the Whin Sill. In the southern half of the district, south of the line of the Roman Wall, where, the inclination of the strata being generally low, the same stratigraphical horizons are exposed by denudation far to the east of the main outcrops of the rocks, we know that the sill must have a subterranean extent of more than 400 square miles. Yet this is probably only a small part of the total area over which the molten material was injected. In the northern part of the district, the Carboniferous Limestone series is not exposed over so broad a stretch of country, and denudation has not there revealed the eastward extension of the sill. But there is no reason to suppose the sheet to be less continuous and massive there. We must remember also that the present escarpment has been produced by denudation, and that the intrusive sheet must have once extended westwards beyond its present limits at the surface. If, therefore, we were to state broadly that the Great Whin Sill has been intruded into the Carboniferous Limestone series over an area of 1000 square miles we should probably be still below the truth. The rock composing this vast intrusive sheet is a dolerite or diabase, which maintains throughout its wide extent a remarkable uniformity of petrographical characters. In this and other respects it illustrates the typical features of sills. Thus it is coarsest in texture where it is thickest, and somewhat finer in grain towards its upper and lower surfaces than in the centre. Among the coarser varieties the component crystals of augite are not infrequently an inch in length and occur in irregular patches.[9] Occasional amygdaloidal portions are observable, but these are not more marked than those to be found in the "whin-dykes" of the same region.[10] The amygdaloidal and vesicular fine-grained rocks of the Bamborough district may possibly be quite distinct from the main body of the Whin Sill.