[6] The Whin Sill has been the subject of much discussion, and a good deal of geological literature has been devoted to its consideration. The writings of Trevelyan, Sedgwick, W. Hutton, Phillips and Tate are especially deserving of recognition. The intrusive character of the Sill, maintained by some of these writers, was finally established by the mapping of the Geological Survey, and was discussed and illustrated by Messrs. W. Topley and G. A. Lebour in a paper in the 33rd volume of the Quart. Journ. Geol. Soc. (1877), in which references to the earlier observers will be found. See also Prof. Lebour's Outlines of the Geology of Northumberland, 2nd edit. (1886), p. 92. The petrography of the Whin Sill is fully treated by Mr. Teall in Quart. Journ. Geol. Soc. xl. (1884), p. 640, where a bibliography of the subject is also given.
From the furthest skerries of the Farne Islands southwards to Burton Fell on the great Pennine escarpment, a distance in a straight line of about 80 miles, this intrusive sheet may be traced in the Carboniferous Limestone series (Map I.). There are intervals where its continuity cannot be actually followed at the surface, but that it really runs unbroken from one end to the other underground cannot be doubted by any one who has examined the region. This singular feature in the geology and scenery of the North of England is known locally as the Great Whin Sill.[7] From the rocky islets and castle-crowned crags of the coast-line it maintains its characteristic topography, structure and composition throughout its long course in the interior. So regularly parallel with the sedimentary strata does it appear to lie, that it was formerly regarded by many observers as a true lava-sheet, poured out upon the sea-floor over which the limestones and shales were laid down. But its really intrusive character has now been clearly demonstrated. Not a vestige of any tuff has been detected associated with it, nor does it ever present the usual characters of a true lava-stream.[8] Its internal structure and the wonderful uniformity in its character mark it out as a typical intrusive sheet.
[7] "Whin" is a common term in Scotland and the North of England for any hard kind of stone, especially such as can be used for making and mending roads. "Sill" denotes a flat course or bed of stone, and was evidently applied to this intrusive sheet from its persistent flat-bedded position and its prominence among the other gently inclined strata among which it lies. It is from this example in the North of England that the word "sill" has passed into geological literature.
[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.
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.
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.