In the soil-clays of the north-west of England there occurs a mineral with a 14 Å basal spacing which resembles vermiculite but differs in important ways. The mineral is generally more abundant in the surface soils than in the lower layers but only in rare cases does it form more than a minor part of the clay fraction. The other minerals present in these clays are almost invariably mica, kaolin, quartz, chlorite and often hydrous iron oxides. In one soil however, from Moses Cocker's Farm, Rivington, Lancashire, the 14 Å mineral forms a major part of the clay fraction of the surface soil. This soil has the laboratory number La 55/4 and will be referred to as such. The mineral also occurs in smaller amounts in the clay fractions of the deeper horizons of this soil and as the surface soil is at least in part derived from the underlying material, two other samples La 55/6 and La 55/9 were also included in the study.

The phenomenon of thixotropy has often been explained by postulating the formation of a lattice structure from the solid particles of the gel. Such a structure is assumed to arise on standing, and to be destroyed on shaking.

The use of preferentially oriented aggregate specimens of clay for X-ray diffraction has been widely adopted since its origination by Bradley, Grim and Clark (1937). It is an extremely useful technique giving greatly enhanced intensity of diffraction from the basal planes of platey minerals relative to that from the general planes; the amount of one particular clay mineral which can be identified in a mixture is very much less than by the normal powder method.

Differential thermal analysis has been used by Hendricks, Nelson and Alexander (1940) to study the sorption of water by montmorillonite. They measured the water uptake at a number of different relative humidities and then traced the low temperature endothermic peak of the resultant material on a differential thermal apparatus. They found that the resultant curves of Mg, Ca, Sr, Ba and Li-montmorillonite showed a division into two components at low relative humidities. Assuming that the area under the D.T.A. curve was proportional to the weight of water lost, a division of the total sorbed water was made between the two peaks. The authors found that for Mg2+, Ca2+, Sr2+ and Ba2+ the high temperature component represented approximately 5—6 water molecules per exchange ion, whilst for Li+ it corresponded to 3·3 water molecules per exchange ion.

Soil formation has been generally attributed to the interaction of five variables: parent material, vegetation, relief, time, and climate. This interaction may be considered as the impact of the latter four forces on the parent material and the resultant is a profile of weathering that varies with environment. The objective of this paper is to demonstrate how the mineralogical characteristics of soil profiles may be used to determine their genesis.