The mineral base of the soils in Islay, Tiree, Coll and the Outer Hebrides is composed mainly of erosion products of Lewisian and Torridonian rocks, together with materials taken from solution in the sea biotically, and subsequently deposited in the form of shell-sand on the windward coasts. The soils composed for the most part of erosion products are acid, and those of marine products basic. Along transect lines at right angles to the coast there is in many places a wide range of soil conditions from calcareous sand to peat, in distances of about 1 kilometre.
The distribution of the Lumbricidæ, a group known to be affected by pH, available calcium content, and moisture content of the soil, is studied within this range in soil types. Seventeen species are involved, three of which are grouped (in numerical considerations) owing to taxonomic difficulties, and these are described in groups living in the main ecological zones of the soil habitat. Relationships are found between the abundance of those species and soil types.
The occurrence of earthworms in three different soil niches—open soil, in and under cow-pats, and under stones—is described, and niche preferences of the dominant species discussed.