Flint clay is kaolinitic mudstone that distinctively differs from widely known shale in lithology, genesis, and economic uses. Flint clay is described as a fine-grained, compact, non-fissile, essentially monomineralic kaolinitic rock that breaks with a conchoidal ("flinty") fracture, has almost no natural plasticity, and resists slaking. It is valuable as a refractory raw material but is also geologically interesting because of its sedimentology, unique in some respects, which is emphasized in this report. Although not particularly abundant in sedimentary rocks, flint clay is worldwide in geographic occurrence. Flint clay is interpreted as having been a product of very early diagenesis--a diagenesis that occurred mainly during accumulation of parent aluminum silicate material rather than dominantly after its deposition and consolidation, the classical concept of diagenesis. The depositional environment typically was non-marine, paludal or fluviatile. at a time of local crustal stability, within the environment and climate typical of Coal Measures. Diagenetic reactions producing flint clay were dominated by desilication, partial removal of iron, alkaline and alkali earths producing a mud that had the composition of kaolin, followed by crystallization or recrystallization of the "digested" sediment into interlocking crystals of kaolinite which is rather well-ordered crystallographically. Where leaching of silica and potassium did not progress far, some illite may have persisted with the kaolinite. Elsewhere, more intense desilication of the flint clay sediment produced diaspora or less commonly boehmite. Flint clay is an intermediate member of the flint clay facies which, landward from marine shale, includes non-marine plastic, semi-plastic, semi-flint, and flint clay, with commonly associated diaspora or boehmite. Sedimentologically associated with it is also sandy flint clay, sandstone, sporadic coal, carbonaceous clay, pyrite, tripolitic chert, and cherty conglomerates. It is doubtful that the process of resilication of high-alumina minerals produced commercial deposits of flint clay.