SILL, in geology, an intrusive mass of igneous rock which consolidated beneath the surface and has a large horizontal extent in comparison with its thickness. In north-eastern England there is a great mass of this kind known as the Whin Sill. "Whin" desig nates hard, tough, dark coloured rocks often of igneous origin, and the Whin Sill is a mass of dolerite or, more strictly, quartz dolerite. Its most striking character is the great distance over which it can be traced. It starts not far north of Kirkby Stephen (Westmorland) and describes a great curve with its convexity towards the west, till it ends on the sea-shore at Bamborough, not far south of Berwick-on-Tweed. It has been pierced in a deep boring at Crook, in Durham. The length of the outcrop is about 8omi., but in places it is covered with superficial deposits or may be actually discontinuous. Near Haltwhistle, however, it is visible for about 2omi., and as it lies among softer rocks (limestones and shales), it weathers out on a bold craggy ridge or escarpment. When it crosses the streams the resistant charac ter of the igneous rock is indicated by waterfalls or "forces," e.g. High Force in Teesdale. The thickness varies from 20 to I soft.
but averages 90 feet. In some places the Whin Sill splits up in to two or more smaller sills which may unite, or one of them may die out and disappear, and often small attendant sills, resembling the main mass in petrographical character, appear in association with it. It is difficult to estimate the area over which it ex tends, as it dips downwards from its outcrop and is no longer visible, but we may conjecture that it spreads over no less than 4,000 sq.mi. underground.
The rocks in which it lies belong to the Carboniferous Lime stone series, and the Sill is probably one of the manifestations of the volcanic activity which occurred during the later part of the carboniferous period. Many similar sills, often of large size, though none so great as the Whin Sill, are found in the Scottish coalfields. There are few lavas or ash beds at or above the horizons on which these intrusive rocks lie, and hence it has been concluded that towards the close of that volcanic episode in British geological history the molten magmas which were im pelled upwards towards the surface found a place of rest usually within the sedimentary rocks, and rarely flowed out as lavas on the sea-bottom (the intrusive succeeding the effusive phase of volcanic action). In the Carboniferous rocks the Whin Sill lies al
most like an interstratified bed, following the same horizon for many miles and hardly varying more in thickness than the sedi mentary bands which accompany it. This, however, is true only on a large scale, for where the junctions are well exposed the igneous rock frequently breaks across the layers of stratification, and sometimes it departs quite suddenly from one horizon and passes to another, where again for a time it continues its apparently regular course. Its intrusive character is also shown by the emis sion of small veins, never very persistent, cutting the sediments above or below it. In addition it bakes and hardens the adjacent rocks, both below and above, and this proves that the superjacent beds had already been deposited and the molten diabase forced its way along the bedding planes, as natural lines of weakness. The amount of contact alteration is not usually great, but the sandstones are hardened to quartzites, the shales become brittle and splintery, and in the impure limestones many new calc silicates are produced.
The Whin Sill consists of dark-green granular diabase, in which quartz or micropegmatite appears as the last product of crystalli zation. It is not usually vesicular and is not porphyritic, though exceptions may occasionally be noted. At both the upper and the under surface the diabase becomes much finer grained, and the finest intrusive veinlets which enter the surrounding rocks may even show remains of a glassy base. These phenomena are due to the rapid cooling where the magma was in contact with the sedi ments. No ash beds accompany the Whin Sill, but there are cer tain dikes which occur near it and probably belong to the same set of injections. In many places the diabase is quarried as a road mending stone.