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The palaeogeographic, tectonic, and biologic conditions of seatearth deposition are briefly reviewed. Detailed mineralogical examination reveals that seatearths are essentially quartz-illite-kaolinite assemblages with minor chlorite, siderite, and organic matter. The illites show wide variation from well ordered sedimentary muscovites to disordered and/or interlayered 'illitic' minerals. The kaolinites are found to be relatively well crystalline and the chlorites are Mg rich. Siderite is commonly associated with minor kaolinite and pyrite in nodules. The organic matter consists of terrestial plant debris. The distribution functions of elements and minerals in seatearths are discussed, A normal model is reasonably satisfactory for SiO(_2), Al(_2)O(_3), CaO, P(_2)O(_5), Ni, Y, and Zr; a lognormal model for TiO(_2), Fe(_2)O(_3), MgO, Na(_2)O, K(_2)O, S, C, H(_2)O(^+), H(_2)O(^-), CO(_2), Ba, Or, Ou, La, Mn, Nb, Rb, Se, Sr, and Zn. Stepwise multiple linear regression is used to assess the relationships between chemistry and mineralogy. It is found that the major components are controlled by the mineralogy to a large extent and that the trace elements and similarly controlled but to a lesser degree. The present data, when compared with previous data, is shown to be comparable .R-mode factor analysis is used to derive factors which control the distributions of trace elements and minerals in seatearths. Eight factors are extracted and they relate to the depositional environment and stratigraphic controls. Roof rocks are examined and the pattern they produce after R-mode factor analysis is shown to be similar to that for a set of clastic cyclothemic sediments. The significance of differences between seatearths and roof rocks are discussed. It is concluded that seatearths are leached relative to their corresponding roof measures. The data is summarised and a detailed theory for the origin and genesis of seatearths, based on that of MOORE (1968), is developed.