O. C. FARRINGTON. The Preterrestrial History of Meteorites, Jour. Geol., vol. 9, 1901, pp. 623-236. E. S. DANA. Minerals and How to Study Them (a book for beginners in mineralogy). Wiley, New York, 1895. On the nature of crystals:— VICTOR GOLDSCHMIDT. Ueber das Wesen der Krystalle, Ostwalds Annalen der Naturphilosophie, vol. 9, 1909-1910, pp. 120-139, 368-419. CHAPTER IV THE ROCKS OF THE EARTH’S SURFACE SHELL =The processes by which rocks are formed.=—Rocks may be formed in any one of several ways. When a portion of the molten lithosphere, so-called _magma_, cools and consolidates, the product is _igneous_ rock. Either igneous or other rock may become disintegrated at the earth’s surface, and after more or less extended travel, either in the air, in water, or in ice, be laid down as a sediment. Such sediments, whether cemented into a coherent mass or not, are described as _sedimentary_ or _clastic_ rocks. If the fluid from which they were deposited was the atmosphere, they are known as _subaërial_ or _eolian_ sediments; but if water, they are known as _subaqueous_ deposits. Still another class are ice-deposited and are known as _glacial_ deposits. [Illustration: FIG. 16.—Laminated structure of sedimentary rock, Western Kansas (after a photograph by E. S. Tucker).] But, as we have learned, rocks may undergo transformations through mineral alteration, in which case they are known as _metamorphic_ rocks. When these changes consist chiefly in the production of more soluble minerals at the surface, accompanied by thorough disintegration, due to the direct attack of the atmosphere, the resulting rocks are called _residual_ rocks. =The marks of origin.=—Each of the three great classes of rocks, the igneous, sedimentary, and metamorphic, is characterized by both coarser and finer structures, in the examination of which they may be identified. The igneous rocks having been produced from magmas, which are essentially homogeneous, are usually without definite directional structures due to an arrangement of their constituents, and are said to have a _massive_ structure. Sedimentary rocks, upon the other hand, have been formed by an assorting process, the larger and heavier fragments having been laid down when there was high velocity of either wind or water current, and the smaller and lighter fragments during intermediate periods. They are therefore more or less banded, and are said to have a _bedded_ or _laminated_ structure (Fig. 16).