The destructive agencies supply a great mass of material, of which, under existing conditions of climate, topography, etc., about one-half is arrested in its journey to the sea and the remaining half completes that journey; the former moiety constitutes the continental deposits, and the latter moiety the terrigenous marine deposits.
Continental deposits are of great variety, and their nature is determined chiefly by the factors of climate and topography. In the arid and desert regions we have great accumulations of drift-sands, of angular talus, of flood-plain and playa sands and muds, which are characteristically sun-cracked and more or less impregnated with various salts. Deposits from salt lakes, such as salt, gypsum, soda, borax, etc., are confined to arid climates and are not formed in humid climates. In pluvial climates of the temperate zones, rain-wash, deep soils, lacustrine deposits from fresh-water lakes, and river deposits on flood plains and in channels are characteristic. In such climates sun cracks do not form over great areas, as they are largely prevented by the dense covering of vegetation. Peat bogs are the seats of great vegetable accumulations, especially in the cooler and moister regions. In the polar regions, glacial deposits and frost talus are the principal modes of accumulation, and in high mountains these also penetrate deep into the temperate and even the tropical zones. In the tropics we find extremely deep soils, which contain or are made up of the red laterite, and surface deposits of iron oxide and chemically formed limestone are extensively made.
Immense masses of river alluvium gather in interior basins, but vegetable accumulations are less abundant than in temperate lands, and lakes are not common in the tropics. The absence of lakes, however, is not determined by temperature, but by the antiquity of land surfaces. It cannot be inferred from the fact that only half of the annual land-waste finds its way to the sea, that such should be the proportion between continental and marine deposits among ancient rocks, for a transgression of the sea over an ancient land surface, deeply buried under continental deposits, would rapidly rework the latter into marine deposits. At present, we observe that material derived from the land and in mechanical suspension laid down in the sea is distributed by the waves and currents, sorted into layers according to the fineness of the material and, more or less incompletely, according to its mineralogical composition. The most important factors which determine the character of the deposit at any given point on the sea-floor are the depth of water and the topography and elevation of the adjoining land.
The coarser material, gravel and sand, are laid down upon the beach and in shoal water, the sand generally extending to the 100-fathom line, while on the continental slope are deposited the various muds, and on the floor of the ocean basins the organic oozes and the oceanic red clay, derived chiefly from the decay of volcanic minerals. Limestone banks are formed by the extraction of the dissolved lime-salts through organic agencies, a process which goes on most extensively in warm seas of shallow and moderate depth. Climatic differences also have their effect upon marine deposits, but less markedly than in the case of the continental accumulations. The loose sediments accumulated on land or under water are, under favouring conditions, consolidated into hard rocks, thus making the parallel with the ancient sedimentary rocks complete, and finishing the cycle of destruction and reconstruction from rock back to rock. All these various kinds of deposits, continental and marine, are forming simultaneously, but one kind of deposit does not gather indefinitely at one point, except perhaps on the floor of the deep ocean-basins. Conditions shift and change, so that one kind of material is laid down upon another, and in the same vertical section we may discover many different beds, each one recording the conditions at that point, for the time during which the bed formed the surface of the lithosphere.
All these changes we have studied in order to obtain a key to the record contained in the rocks, and we have found that the processes now at work do furnish a partial key. However, before a systematic history of the earth can be attempted, we have first to study the ways in which the rocks are arranged and the disturbances which they have undergone; this constitutes structural or tectonic geology.