4 Limestone country dissected by ravines, Hercegovina 51 5 Sand developing from Sandstone, Cape of Good Hope 59 6 Siliceous Conglomerate, Co. Waterford 75 7 Quartzite Cone, Croagh Patrick 77 8 Shrinkage-cracks in Clay, Spitsbergen 81 9 Landslide of Limestone over Shale, Drôme 93 10 Weathering of Shale, Isère 95 11 Boulder-clay, Crich, Derbyshire (_Phot._ H. A. Bemrose) 97 12 Nordenskiöld Glacier, Spitsbergen 99 13 Sefström Glacier, Spitsbergen 101 14 Ash-layer of 1906 on Vesuvius 111 15 Puy de la Vache, Puy-de-Dôme 113 16 Granite invading Mica-schist, Cape Town 121 17 Weathering granite, Lundy Island 139 18 Granite weathering under tropical conditions, Matopo Hills 141 19 Composite Gneiss, Co. Donegal 153 20 Composite Gneiss, Ängnö, Sweden 155 (Figs. 11 and 17 are reproduced from the Cambridge County Geographies of _Derbyshire_ and _Devonshire_ respectively; the rest of the illustrations are from photographs by the author.) CHAPTER I ON ROCKS IN GENERAL The description of rocks has fallen very much into the hands of lovers of analysis and classification, and attention has been diverted, even among geologists, from their fundamental importance as parts of the earth's crust. The geographer or the general traveller may often wish for closer acquaintance with the units that build up the scenery around him. The characters of rocks again and again control the features of the landscape. When studied more nearly, these same characters imply conditions of deposition or solidification, and lead the mind back to still older landscapes, and to the meeting of oceans and continents on long-forgotten shores. Petrology, indeed, involves the understanding of how rocks "come to be where we find them when we try"; but the classification of hand-specimens was from the first easier than field-investigation, and in later times the science was threatened with the description of isolated microscopic slides. Fortunately, a certain amount of feeling for natural history has been imported again into the subject, and evolutionary principles and sequences have been discussed. Experimental work, moreover, has been brought to bear on the question of the origins of rocks, with more success than might have been expected, since it is very difficult to realise in a laboratory, or even in the mind, the conditions that prevail in the lower parts of the earth's crust.