According to de Lapparent (Trait* de GeĀ¢logie), the trickling of rain-water, when very abundant, degrades the neighbouring slopes and carries down, according to the strength of the flow, sometimes fine soil and sometimes fragments of stone; this would be sufficient to account for the formation of lehm, which is therefore also called tableland slime. This trickling, many times repeated, was produced by streams of water thin enough to allow free access to the air; that is to say, the formation of the slime took place in an oxidising atmosphere, whence this peculiarity that the clay contains no organic substances, and that the iron in it is, for the most part, in the form of peroxide, as is shown by the yellow colour of the mass.
We know that glacier-earths and river-mud, which are similar to lehm, but are formed without access of air, have a bluish grey colour, due to the organic matter and the nature of the iron-salts contained in them.
As for the red slime which, showing signs of having been hollowed out by water, covers the deposits of lehm, and forms the beetroot soil of the North, it is also employed for the manufacture of brick, but opinions are divided as to its origin. It contains no limestone, but in many places contains angulous splinters of white oxidised flint.
According to Wood, the red slime with splintered flint is the result of the alternation of frost and thaw on the surface of the earth; the changes of temperature lightened the crust and splintered the flints, which, being situated in a frequently muddy mass, acquired the white oxidisation, a sign of change. This transformation, taking place at different depths, must have produced those signs of hollowing which are observed on the horizontal parts, while on the inclined parts there was a real hollowing-out, due to the slipping of the slime which was reduced to pulp by its mixture with snow.
It is known that sedimentary rocks are divided into four groups according to the age of their formation: primary, secondary, tertiary, and quaternary or modern period. Each group is subdivided into systems which form stages, according to the age of their deposit.
In all these systems clays are found, even in the primary. We will point out to which geological stages the principal clay deposits belong.