In lithology we investigate the nature of the materials composing the earth’s crust—the various minerals and aggregates of minerals, or rocks; while in petrology we consider the forms and modes of arrangement of the rock-masses,—in other words, the architecture of the earth. Petrology is the complement of lithology, and in many respects it is the most fascinating division of geology, since in no other direction in this science are we brought constantly into such intimate relations with the beautiful and sublime in nature. The structures of rocks are the basis of nearly all natural scenery; for what we call scenery is usually merely the external expression, as developed by the powerful but delicate sculpture of the agents of erosion—rain and frost, rivers and glaciers, etc.—of the geological structure of the country. And to the practised eye of the geologist, a fine landscape is not simply a pleasantly or grandly diversified _surface_, but it has _depth_; for he reads in the superficial lineaments the structure of the rocks out of which they are carved. But, while the magnitude of the phenomena adds greatly to the charm of the study, it also increases the difficulties and taxes the ingenuity of the teacher whose work must be done indoors. According to our ideal method, natural science ought to be taught with natural specimens; and yet here our main reliance must be upon pictures and diagrams. Nature, however, has not been wholly unmindful of our needs; for she has worked often upon a very small as well as a very large scale; many of the grandest phenomena being repeated in miniature. Thus we observe rock-folds or arches miles in breadth and forming mountain masses, and of all sizes from that down to the minutest wrinkle. So with veins, faults, etc. And the wonderful thing is that these small examples, which may be brought into the class-room, are usually, except in size, exactly like the large. Now the aim of every teacher in this department should be to secure a collection of these natural models. It is not an easy thing to do, except one has plenty of time; for they can rarely be purchased of dealers, but must usually come as the choicest fruit of repeated excursions to the natural ledges and quarries, the seashore and the mountains. But for the difficulty of getting the specimens there is some compensation, since it may be truly said that for the _collector_ specimens obtained in this way have an interest, a value, and a power of instruction beyond what they would otherwise possess. _Classification of Structures._