The deposits of wave-swept beaches leave us _Conglomerates_ formed of various types of pebbles, among which quartz-rock and quartzite naturally predominate. In some cases the pebbles are ready formed when they reach their resting-place. They come rolling out from lateral torrents into the quieter waters of a main valley, as may be seen in summer in the broad pebble-banks of the north Italian streams. Thence they are washed by occasional floods into the great confluent deltas that constitute the upper part of an alluvial plain, or into lake-basins, where they promptly settle along the shore. But few such pebbles, except from pre-existing conglomerates or gravels on the shore-line, actually reach the sea. The rolled stones upon sea-beaches are mostly the products of marine action on the spot. While the fine sand-grains go seaward almost unharmed, the detrital stones, offering far less surface in proportion to their mass, strike on their neighbours as every wave shifts them on the beach, and soon assume a rounded form. The conglomerates ultimately consolidated may reveal stratification only by the general arrangement of their pebbles. These can rarely be spheres, since they are not as a rule turned over, but are pushed this way and that until they acquire a flat ellipsoidal shape. They lie with their flatter sides in planes parallel to one another. Generally, however, alternations of coarser and finer beds mark out the stratification even in conglomerates. The sands of deserts include abundant stones and blocks of rock, and the loose material becomes, moreover, sifted by the wind. True desert sands may accumulate at one point, the very finest loamy material may be carried away still farther to form fields of fertile _löss_, and a rock-desert, formed of stones resting on bare surfaces, may remain in large areas of the arid region. The loose stones here assume a characteristic shape, and have been known under the German name of _Dreikanter_. They are fairly flat below, and are cut away above by the drifting sand into a form resembling a gable roof dipping at both ends. Their surfaces are characteristically etched. _Dreikanter_ have been found in beds that were formerly ascribed to deposition on the shores of lakes, and it must now be borne in mind that continued attrition by drifting sand affects mixed detritus on a land surface much as the wash of waters does upon a beach. Certain materials are cut away more rapidly than others, and the residue assumes a more and more quartzose type. In this way, sandstones, and conglomerates in which fragments of quartzite and vein-quartz predominate over other constituents, may arise as æolian beaches on dry land. SANDSTONE AND THE LAND-SURFACE