The factor of grade size is the one most used for the classification of sedimentary rocks (p. 190). Four groups may be distinguished:— 1. Rudaceous 1—Rocks consisting chiefly of gravel, pebbles, cobbles, or boulders. Loose materials of this class are gravels, pebble-beds, shingle, boulder-beds, scree, talus, etc. When cemented they form conglomerates and breccias. 2. Arenaceous—Rocks consisting chiefly of material of sand grade. Loose materials are sands; when consolidated they form sandstones, grits, arkoses, graywackes, etc. 3. Silt Rocks—Rocks consisting chiefly of material of silt grade. Silt and siltstones. This class is usually included either with the arenaceous or the argillaceous classes, but several distinct rock types occur in it, and the group is worthy of a separate designation. 4. Argillaceous—The clay rocks, consisting of the finest materials of rock decay. Dust, mud, clay, when more or less unconsolidated; mudstone and shale when compacted.