Agricultural geology . by its own kineticenergy. The form in this case will be largely determined bythe disposition of the planes of weakness, of the divisionalplanes, in the surrounding rocks. I] MINERALS AND ROCKS 13 The simplest case of all, perhaps, is when the molten materialmerely fills a crack in the ground, vertical or otherwise. Itmay or may not reach the surface, according to a mass is called a dyke. If the crack reaches the surfacemolten material may spread out as a thin sheet, just as with alava-flow; in fact one well-known type of volcanic eruption,the fiss
Agricultural geology . by its own kineticenergy. The form in this case will be largely determined bythe disposition of the planes of weakness, of the divisionalplanes, in the surrounding rocks. I] MINERALS AND ROCKS 13 The simplest case of all, perhaps, is when the molten materialmerely fills a crack in the ground, vertical or otherwise. Itmay or may not reach the surface, according to a mass is called a dyke. If the crack reaches the surfacemolten material may spread out as a thin sheet, just as with alava-flow; in fact one well-known type of volcanic eruption,the fissure-eruption, is exactly of this kind; lava wells upthrough a crack and spreads out on the surface; the dyke inthis case is the feeder of the flow. Dykes are not necessarilyvertical at first and may subsequently be tilted at any name is usually restricted to intrusions which clearly cutacross the bedding or foliation planes of the surrounding the molten material forces its way as a sheet of varying. Fig. .3. A vei-tioal dyke passing up into a sill which has penetratedalong the bedding planes of the sedimentary rooks. thickness along the bedding planes of a sediment it is called asill. Similarly, more or less horizontal masses cutting acrossfolded and crumpled strata are called sheets. The extent andthickness of sills vary indefinitely; some are known to extendover hundreds or even thousands of square miles, the WhinSill of the north of England and the Palisade Traps of NewYork. In some parts of the world, where sills are very abundant,they are the determining features in the topography and hencein the economic value of the land. In the stratified rocksforming the Great Karroo in South Africa, sills are extremelyabundant, and by their superior hardness they give rise to the 14 INTRODUCTION [CH. peculiar kopje type of scenerV: consisting of steep, terracedand often flat-topped hills, with deep valleys between. Thekopjes are rocky and barren, but the vall
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