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Sandstone: sedimentary rock type, formed of a lithified sand, comprising grains between 63 μm and 1000 μm in size, bound together with a mud matrix and a mineral cement formed during burial diagenesis. The main constituents are quartz, feldspar, mica, and general rock particles, although the proportions of these may vary widely. Argillaceous: applied to rocks which are silt- to clay-sized sediments (grain size less than 0.0625 mm in diameter). They account for more than 50% of sedimentary rocks and most have a very high clay mineral content. Many contain a high percentage of organic material and can be regarded as potential source rocks for hydrocarbons. — A Dictionary of Geology and Earth Sciences (4 ed.), Oxford University Press