The Plant Ecology of Serpentine: I. Serpentine Vegetation of England and Scotland

tively; it also includes chemical soil analyses. In this paper an attempt is made to relate British serpentine vegetation to that in other parts of the world, and to provide a background for the experimental work to be presented in later papers. Serpentine is an imprecise term. Even in the strict mineralogical sense 'serpentine' refers not to one but to a group of minerals. These are usually hydrothermally altered products of olivine (MgFe)2 SiO4 and have the approximate composition H4 (MgFe)3Si209. The chemical composition of individual serpentines varies enormously, however, and some are derived from the calcium-containing mineral augite (CaMg(Fe)Si206). Petrologically, 'serpentine' is frequently used for 'serpentinite' to denote the rock consisting primarily of the mineral. Serpentine rocks can contain many associated minerals. These are often hydrated magnesium rich silicates, e.g. tremolite, or they can be residual unaltered minerals, e.g. chromite. The rock belongs to the 'ultrabasic' group which is characterized by low silicon and relatively high magnesium and iron contents. Table 1 illustrates the chemical composition and compares this with that of 'basic' gabbro and 'acidic' granite. The chemical composition of soils derived from serpentine rocks varies greatly: for instance, the eight soils analysed by Ferreira (1963) and our own analyses for Rhum, Meikle Kilrannoch and the Lizard, in Tables 4 and 6, illustrate a wide range of calcium and magnesium contents. The ecological definition of serpentine is even less precise, including unaltered olivine and peridotite. This wide definition has its merits because of the chemical similarities