Agricultural geology . some parts of the world, Canada, India and SouthAfrica, are found masses of igneous rock, especially of granite,covering hundreds or thousands of square miles, and of unknowndepth. Such masses can hardly have been intruded in theordinary sense of the word; they have more probably beenformed by bodily fusion in situ of pre-existing rocks while undera very thick cover of later rocks, during a period of depressioninto a hotter region of the earth. A fairly common special form of igneous mass is theneck. This is in point of fact nothing but the material fillingthe lower
Agricultural geology . some parts of the world, Canada, India and SouthAfrica, are found masses of igneous rock, especially of granite,covering hundreds or thousands of square miles, and of unknowndepth. Such masses can hardly have been intruded in theordinary sense of the word; they have more probably beenformed by bodily fusion in situ of pre-existing rocks while undera very thick cover of later rocks, during a period of depressioninto a hotter region of the earth. A fairly common special form of igneous mass is theneck. This is in point of fact nothing but the material fillingthe lower part of the pipe or vent of a volcano, which has I] MINERALS AND ROCKS 15 solidified, and has been subsequently exposed at the surface bythe removal of the overlying rocks. In consequence of theirsuperior hardness, necks often stand up as elevations above thesofter rocks of the surrounding country; such are many well-known hills in the central lowlands of Scotland, NorthBerwick Law, Largo Law, Arthurs Seat, Pig. 5. A volcanic neck which has been denuded and nowstands up to form a hill. Besides the more or less definite forms mentioned above,igneous rocks often occur in shapeless masses of every possiblesize, their forms depending for the most part on the arrangementof the planes of weakness in the surrounding rocks. An extremecase is where molten material has been injected in numerousthin sheets along the bedding planes of sediments or the fohationplanes of metamorphic rocks. This constitutes the so-calledlit-par-lit injection, which is common in many areas of ancientcrystalline rocks, giving rise to banded gneisses. Divisional planes in rocks. When any exposure of rocks isexamined, whether natural or artificial, perhaps the first thingto be noticed is that the masses are not continuous. In almostevery case, the rocks are broken up by planes of discontinuityinto natural blocks of very various shapes and sizes. Thesedivisional planes are of several different kinds an
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Keywords: ., bookcentury1900, bookdecade1910, bookidcu3192, booksubjectgeology
