FIGURE LIST OF ILLUSTRATIONS 67. Section of Coal-measures near Cambusnethan, Lanarkshire, on a true scale. xiii PAGE 166 68. 69. Section on a true scale across "Tynedale Fault," Newcastle Coal-field 168 169 70. 71. Section across Great Fault bounding the Southern Uplands 72. 73. 75. 74. Plain of Granite overlooked by Mountains of Schists, etc. Diagrammatic Section of a Laccolith showing Dome-shaped Elevation of Surface above the Intrusive Rock (after G. K. Gilbert) View of Necks-Cores of old Volcanoes (Powell) 76. Section of Highly Denuded Volcano, Minto Hill, Roxburgshire Diagrammatic Section across the Valley of the Tay, near Dundee View of Mesa Verde and the Sierra el Late, Colorado (Hayden's Report for 1875) 77. 78. Wind Erosion: Table-Mountains, etc., of the Sahara (Mission de 254 80. Wind Erosion: Harder Beds amongst inclined Cretaceous Strata, 82. Manganese Concretions weathered out of Sandstone, Arabah Mountains, Sinai Peninsula (J. Walther). Longitudinal Sections of Lake-basins on a true scale 293 319 Plate I. Joints in Granite, Glen Eunach, Cairngorm (from a photograph to face 200 to face 202 EARTH SCULPTURE CHAPTER I INTRODUCTORY EARLY VIEWS AS TO ORIGIN OF SURFACE-FEATURES-ROCKS AND ROCK-STRUCTURES-ARCHITECTURE OF THE EARTH'S CRUST -GENERAL EVIDENCE OF ROCK-REMOVAL. WH HEN geologists began to inquire into the origin of surface-features, they were at first led to believe that the more striking and prominent of these had come into existence under the operation of forces which had long ago ceased to affect the earth's crust to any marked extent. It is not hard to understand how this conception arose. The earlier observers could not fail to be impressed by the evidence of former crustal disturbances which almost everywhere stared them in the face. Here they saw mountains. built up of strangely fractured, contorted, and jumbled rock-masses; there, again, they encountered the relics of vast volcanic eruptions in regions now practically free from earth-throes of any kind. In one place ancient land-surfaces were seen intercalated at inter vals among great successions of marine strata; in other places, limestones, evidently of oceanic origin, were found entering into the framework of lofty mountains far removed from any sea. It was these and similar striking contrasts between the present and the past which doubtless induced the belief that the earth's crust, after having passed through many revolutions more or less catastrophic in character, had at last become approximately stable-the occasional earthquakes and volcanic disturbances of recent times being looked upon as only the final manifestations of those forces which in earlier ages had been mainly instrumental in producing the varied configuration of the land. Mountains and valleys belonged to earth's Sturm und Drang period. That wild time had passed away, and now old age, with its lethargy and repose, had supervened. The tumultuous accumulations of stony clay, blocks and boulders, gravel and sand that overspread extensive areas in temperate latitudes were believed to be the relics of the last great catastrophe which had affected the earth's surface. Some notable disturbance of the crust, it was thought, had caused the waters of northern seas to rush in devastating waves across the land. When these diluvial waters finally retired, then the modern era began an era characterised by the more equable operation of nature's forces. But with increased knowledge these views gradually became modified. Eventually, it was recognised that no hard-and-fast line separates past and present. The belief in world-wide, or nearly world-wide, catastrophes disappeared. Geologists came to see that the fashioning of the earth's surface had been going on for a long time, and is still in progress. The law of evolution, they have found, holds true for the crust of the globe just as it does for the myriad tribes of plants and animals that clothe and people it. It is no longer doubted that the existing configuration of the land has resulted from the action of forces that are still in operation, and by observation and reasoning the history of the various phases in the evolution of surface-features can be unfolded. No doubt the evidence is sometimes hard to read in all its details, but its general bearing can be readily apprehended. The salient facts, the principal data, are conspicuous enough, and the mode of their interpretation is in a manner self-evident. In setting out upon our present inquiry, however, it is obvious that we ought, in the first place, to know something about rocks and the mode of their arrangement. We must make some acquaintance with the composition and the structure or architecture of the earth's crust before we can form any reasonable conclusion as to the origin of its surface-features. Now, so far as that crust is accessible to observation, it is found to be built up of two kinds of rock, one set being of igneous origin, while the other appears to consist mainly of the products of water action. These last are typically represented by such rocks as conglomerate, sandstone, and shale, which are only more |