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Encyclopedia Britannica - Main :: SHA-SIV |
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SILL , in geology, an intrusive mass of igneous rock which consolidated beneath the surface and has a large horizontal
ridge
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 impelled 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 almost like an interstratified bed, following the same horizon for many miles and hardly varying more in thickness than the sedimentary 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 emission 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 diaoase 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 a dark-green granular diabase, in which quartz or micropegmatite appears as the last product of crystallization. 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 withthe sediments. No ash beds accompany the Whin Sill, but there are certain 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. The great Palisade trap of the Hudson river, which is an almost exact parallel to the Whin Sill, is an enormous sheet of igneous rock exposed among the Triassic beds of New Jersey
York
Jersey
These great sheets of igneous rock intruded into cold and nearly horizontal
In the Tertiary volcanic district
ridge
In the Tertiary volcanic series of Scotland and Ireland intrusive sheets build up a great part of the geological succession. They are for the most part olivine-basalts and dolerites, and while some of them are nearly horizontal, others are inclined. Among the lavas of the basaltic plateaus there is great abundance of sills, which are so numerous, so thin and so nearly concordant to the bedding of the effusive rocks that there is great difficulty in distinguishing them. As a rule, however, they are more perfectly columnar, more coarsely crystalline and less vesicular than the igneous rocks which consolidated at the surface. These sills are harder and more resistant than the tuffs and vesicular lavas, and on the hill slopes their presence is often indicated by small vertical steps, while on the cliff faces their columnar jointing is often very conspicuous.On modern volcanoes intrusive sheets are seldom visible except where erosion has cut deep valleys into the mountains and exposed their interior structure. This is the case, for example, in Ireland, Teneriffe, Somma and Etna and in the volcanic islands of the West Indies. In their origin the deep-seated injections escape notice; many of them in fact belong to a period when superficial forms of volcanic action have ceased and the orifices of the craters have been obstructed by ashes or plugged by hard crystalline rock. But in the volcanoes of the Sandwich Islands the craters are filled at times with liquid basalt which suddenly escapes, without the appearance of any lava at the surface. The molten rock, in such a case, must have found a passage underground, following some bedding plane or fissure, and giving rise to a dike or sill among the older lavas or in the sedimentary rocks beneath. Many of the great sills, however, may have been connected with no actual volcanoes, and may represent great supplies of igneous magma which rose from beneath but never actually reached the earth's surface. The connexion between sills and dikes is very close; both of them are of subterranean consolidation, but the dikes occupy vertical or highly inclined fissures, while the sills have a marked tendency to a horizontal position. Accordingly we find that sills are most common in stratified rocks, igneous or sedimentary. Very frequently sills give rise to dikes, and in other cases dikes spread out in a horizontal direction and become sills. It is often of considerable importance to distinguish between sills and lavas, but this may be by no means easy. The Sciur of Eigg is a good example of the difficulty in identifying intrusive masses. Lavas indicate that volcanic action was going on contemporaneously with the deposit of the beds among which they occur. Sills, on the other hand, show only that at some subsequent period there was liquid magma working its way to the surface. (J. S. F.) End of Article: SILL If you wish, you can link directly to this article.
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