When the beds remain fairly level, and are also uplifted, flat-topped hills are formed by the intrusive sheets, like those that may be carved out of a country flooded over by lava-streams. The crystalline rock, very probably a dolerite, protects what lies below it. The kopjes north of the Great Karroo in the centre of the Cape of Good Hope are thus level on the crest and bounded by a steep wall or _krans_ of rock. The edges of similar "sills" of igneous rock have controlled much of the scenery between the Highland border of Scotland and the Tyne. A fine example is the indented scarp of the Great Whin Sill, a sheet of dolerite intruded among the Carboniferous strata of Northumberland. This mass forms a platform for Bamburgh Castle against the wild North Sea, and is traceable south-westward across the country towards Carlisle. North of Hexham, its escarpment is occupied by Hadrian's wall, and the town of Borcovicus was planted on the edge, overlooking all Northumbria. The farmers of North Britain and Ireland have long known upstanding igneous dykes as unprofitable "whinstones." The regularity of direction among dykes over very wide areas points to their intrusion along cracks produced by stretching of the crust. Radial grouping of dykes, such as one finds near volcanic necks, or, on a gigantic scale, round Tycho on the moon, may be due to explosive action; but the majority of dykes seem to have followed upon earth-movement. In the north of Ireland, from the coast of Down to that of Donegal, a series of compact rocks of Devonian age occurs in dykes lying almost invariably north and south. The post-Cretaceous dykes of the same region have a still more uniform trend, from north-west to south-east. Such series of dykes modify the scenery of coasts by forming promontories and serviceable piers for boats. The offshoots near the surface of a great intrusive mass are far less regular. We are here close to the zone of attack, the "shatter-zone," and the structures or regular fracture-planes of the overlying rock only partially control the position taken up by the intrusive magma. Irregular knots and bosses appear in place of far-spreading sheets, and a network of crossing veins occurs, instead of a system of coordinated dykes. The resulting country is hummocky and broken, and, where the cauldron itself has become exposed, striking contrasts of surface are seen as we pass from the igneous core to the older and frequently stratified rocks upon its flanks. Some large bodies of intrusive rock have, however, been formed sheet by sheet, and a bedded sill-like structure is then revealed in them on weathering. Sir A. Geikie[89] calls attention to this in his description of the heart of the black gabbro mass in Skye. But, as a rule, the continuity of structure in batholites, and their characteristic joint-planes set at angles to one another, cause them to appear as massive blocks in the landscape, untraversed by any regular lines.