STABLE AND MIGRATORY PLANT FORMATIONS 3 
plant formation as follows :—“A plant formation comprises 
the progressive associations which culminate in one or more 
stable or chief associations, and the retrogressive associations 
which result from the decay of the chief associations, so long 
as these changes occur on the same habitat ” (6). 
The crucial point in this definition is the meaning of the 
term ‘the same habitat,’ since in most cases of known 
progression of associations the habitat suffers change during 
the various stages of succession. The meaning of “the same 
habitat ’’ in this definition is made somewhat clearer in the 
“Types of British Vegetation,” where the master-factor in- 
volved in plant formation is considered to be the nature of 
the soil. In the case of certain formations therein described 
(as, eg., I, the plant formation of clays and loams; 2, 
the plant formation of sandy soil; 3, the plant formation 
of older siliceous soils; 4, the plant formation of cal- 
careous soils) a wide margin of soil difference is allowed 
within the meaning of “the same habitat’’ which defines the 
plant formation, since the very different degrees of water 
content, aeration, humus content, warmth, etc., due to 
drainage, exposure, and depth of soil, ze. the physiography 
of these formations, has as yet received but little attention. 
Clays, loams, and sands, generally speaking, form level or 
gently undulating ground on valley floors, plains, or plateaux. 
In humid regions not recently glaciated all extensive surfaces 
of this nature are cloaked by these deposits, irrespective of 
the nature of the underlying rocks. This is due to clays and 
sands, pure or in varying mixtures, forming the ultimate pro- 
ducts of decomposition of all rocks where the surface has 
long been subject to decay without erosion, and further 
forming the products of accumulation in areas subject to 
sedimentation. Wherever the ground slopes, clays and sands 
are removed by denudation. 
In large continental areas they cloak plains and plateaux 
at very different altitudes, with a wide range in climatic 
conditions, and correspondingly wide changes in the vegeta- 
tion. In the British Isles, as in all smaller areas of fairly 
mature physiography, such materials are, however, chiefly 
confined to lower levels, and the plant formations cover- 
ng them might well be termed collectively the plant forma- 
