Page:Popular Science Monthly Volume 54.djvu/870

 have been made interested in geology by some of these masterly picturesque descriptions of regions with which they were superficially familiar. Other treatises on the origin of surface features, dealing with the subject more fundamentally, but likewise of limited scope, are not wanting. Yet, as Prof. James Geikie well says, there is no English work to which readers not skilled in geology can turn for a general account of the whole subject. Professor Geikie has therefore prepared his elaborate book on Earth Sculpture to supply this want, to furnish an introductory treatise for those persons who may be desirous of acquiring some broad knowledge of the results arrived at by geologists as to the development of land forms generally. A vast number of geological questions are involved in the exhaustive treatment of the subject. All the forces with which geologists become acquainted in the study of the earth, and their operation, come into consideration. The effects of these forces assume aspects that vary according to the nature of the material on which they operate, and they are again modified according to the peculiar combinations of forces at work. The subject is therefore not the easy one it may be supposed at first sight to be, and the reader who peruses Professor Geikie's work with the intention of mastering it will find he has some studying to do. Yet Professor Geikie is clear, and it is only because he has gone deeper than the others that he may be harder. The first point he insists upon is that in the fashioning of the earth's surface no hard-and-fast line separates past and present. The work has been going on for a long time, and is still in progress, under a law of evolution as true for the crust of the globe as for the plants and animals. In setting out upon our inquiry we must in the first place know something about rocks and the mode of their arrangement, of the structure or architecture of the earth's crust. This leads to the distinction between the igneous and the subaqueous, the volcanic, plutonic, and metamorphic, and the derivative rocks on which epigene agencies have performed their shaping work. These rocks have been modified in various ways, and the surface appearance of the earth has been affected by forces operating from the interior, and by external factors, the work of which is called denudation. The agents of denudation are described—air, water, heat, frost, chemical action, plants, and animals—often so closely associated in their operations that their individual shares in the final result can hardly be determined. The various influences of these factors as exerted upon different forms of geological structure and different sorts of rocks are then taken up and described as applied to land forms in regions of horizontal, or gently inclined, and of highly folded and disturbed strata, and in regions affected by normal faults or vertical displacements. Land forms due directly or indirectly to igneous action and the influence of rock character on the determination of land forms are subjects of special chapters. Glacial action is one of the most important factors in modifying the forms of northern lands, and is treated with considerable fullness. Æolian action—of the air and wind—has peculiar and important effects in arid regions, and underground water in limestone districts, and these receive attention. Then come basins—those due to crustal deformation, crater lakes, river lakes, glacial basins, and others, and coast lines. Finally, a classification is given of these land forms as plains or plateaus of accumulation and of erosion, original or tectonic and subsequent or relict hills and mountains, original or tectonic and subsequent or erosion valleys, basins, and coast