[Footnote 8: On the coast at Bamborough and the Harkess Rocks the usual petrographical characters of the Whin Sill are exchanged for those of fine-grained amygdaloidal diabases arranged in distinct sheets, which in their upper parts are highly vesicular and show ropy surfaces--peculiarities suggestive of true lava-streams. But according to Professor Lebour the rocks are intrusive into limestone and shale (_Geology of Northumberland and Durham_, p. 98). Mr. Teall has expressed the suspicion that these rocks must have consolidated under conditions somewhat different from those which characterized the normal Whin Sill (_Quart. Journ. Geol. Soc._ xl. p. 643). They seem to be the only parts of the sill which present features that might possibly indicate superficial outflow.] Among the manifestations of the subterranean intrusion of igneous rocks in the British Isles the Great Whin Sill, next after the Dalradian sills of Scotland, is the most extensive. Its striking continuity for so great a distance, and the absence around it of any other trace of igneous action, save a few dykes, place it in marked contrast to the ordinary type of Carboniferous sills. The occasional gaps on its line of outcrop in the northern part of its course do not really affect our impression of the persistence of the sheet. They not improbably indicate merely that in its protrusion it had a wavy irregular limit, which in the progress of denudation has occasionally been not yet reached. For mile after mile the sill has been mapped by the Geological Survey in lines of crag across the moorlands, and as a conspicuous band among the limestones and shales that form the steep front of the Pennine escarpment, where it has long been known in the fine sections exposed among the gullies by which that noble rock-face has been furrowed. [Illustration: Fig. 176.--Section from the great Limestone escarpment on the west to the Millstone Grit hills east of Teesdale. 1. Silurian strata; 2. Carboniferous Limestone series; 3. The Great Whin Sill, which gradually rises to higher stratigraphical position as it goes westward; 4. Millstone Grit.]