zone in which these changes are at a maximum is called the _zone of weathering_. This general zone is principally above the surface or level of the ground-waters, but for some rocks it extends well below this level. In some regions the ground-water level may be nearly at the surface, and in others, especially where arid, it may be two thousand or more feet down. Disintegrated weathered rocks form a blanket of variable thickness, which is sometimes spoken of as the residual mantle, or "mantle rock." [Illustration: FIG. 1. Graphic representation of volume change in weathering of a Georgia granite.] Mineral products formed by weathering from common igneous rocks include soils, clay, bauxite, and certain iron, chromite, and nickel ores. Again the commercial importance of this group is not large, as compared with products formed in other ways described below. The same weathering processes described above for igneous rocks cause considerable changes of economic significance in deposits formed as igneous after-effects. In some cases they result in removing the less valuable minerals, thus concentrating the more valuable ones, as well as in softening the rock and making it easier to work; and in other cases they tend to remove the valuable constituents, which may then be redeposited directly below or may be carried completely out of the vicinity. The _oxide zones_ of many ore bodies are formed by these processes. SEDIMENTARY PROCESSES Sedimentary rocks are formed by the removal and deposition of the weathered products of a land surface. Air, water, and ice, moving under the influence of gravity and other forces, all aid in this transfer. The broken or altered rock materials may be merely moved down slopes a little way and redeposited on the surface, forming one type of _terrestrial_ or _subaerial deposits_, or they may be transferred and sorted by streams. When deposited in streams or near their mouths, they are known as _river_, _alluvial_, or _delta deposits_. When carried to lakes and deposited they form _lake deposits_. Ultimately the greater part of them are likely to be carried to the ocean and deposited as _marine sediments_. Part of the weathered substances are carried mechanically as clay and sand, which go to make up the _shale_ and _sandstone_ sediments. Part are carried in solution, as for example lime carbonate and magnesium carbonate, which go to make up _limestone_ and _dolomite_. Some of the dissolved substances are never redeposited, but remain in solution as salts in the sea, the most abundant of which is sodium chloride. Some of the dissolved substances of weathering, such as calcite, quartz, and iron oxide, are carried down and deposited in openings of the rocks, where they act as cements.