FORMATIONS AND ASSOCIATIONS _ 33 
and the one is related to the limestone vegetation of 
the hill above, the other to the vegetation of the 
loams and peaty soils of the adjoining mountain-side. 
In the British Isles the leading plant formations are 
those of clays and loams, of sands and sandstones, of 
siliceous soils, of calcareous soils, of peat, of marsh, 
of lakes and rivers, of salt-marsh, sand dune, and 
shingle beach; also, governed by the climatic factor, 
alpine vegetation stands somewhat apart. While the 
vegetation of some of these, such as salt-marsh or 
peat, usually presents a uniform aspect, others, such 
as the clays, sands, and limy soils, display each a 
characteristic type of woodland and of grassland, as 
well as other variants, dependent on the composition, 
depth, and wetness of the soil, the degree of ex- 
posure, and so on: these form the associations which 
together constitute the formation. Each association, 
if the plants composing it be examined, will be found 
to consist of an assemblage of species, large and 
small, brought together by their superior fitness for 
the particular conditions which prevail. There are 
mostly in each association one or more dominant 
species—such as the trees of an Oak wood, or the 
Heather of a moor—which by their abundance or 
vigorous growth control the association. The shelter 
which they give may protect some of the members of 
the community: the shade which they cast may keep 
out other plants which otherwise would invade the 
ground. The association will include some species 
specially adapted to the particular conditions which 
prevail, and perhaps not found elsewhere in the 
area; these are the indicator plants of the association, 
which give it its special character, and which will 
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