MOORLAND ASSOCIATIONS 249 
3. At any altitude where lime is quite absent from the 
soil, as on dry, sandy, or pebbly heaths, where any lime 
at one time present has been washed into the subsoil. 
Peat seldom accumulates directly on limestone or 
chalk, for here the conditions are all in favour of bacterial 
action. Plenty of oxygen and lime is present, and the 
soil is warm, except at very high altitudes. When peat 
does form in a limestone district, it can usually be traced 
tc the presence of a capping of some other material on 
the limestone. . The great bogs of Ireland, for example. 
lie chiefly on limestone covered with boulder-clay. 
The plant-remains in moorland-peat, in contrast to 
those in marshy peat, are well preserved. A section 
through a thick deposit will often show a well-marked 
stratification of the material from which the peat is 
formed. At one level Sphagnum may predominate, at 
another cotton-grass, whilst remains of birch and pine 
are frequent. From these different layers we can gain, 
not only an idea of the former vegetation, but also of the 
changes in climate which the district has undergone. 
Humous acids are formed in all peaty soils, and, if 
present in large quantities, the absorption of water is 
rendered difficult, and the vegetation is of a pronounced 
xerophytic type, as on damp heather-moors and cotton- 
grass bogs. Here the dominant plants are either heath- 
like plants with small evergreen rolled leaves (Figs. 9, 12), 
or grass-like plants with erect cylindrical leaves. Mineral 
food is scarce, especially the nitrates and phosphates of lime, 
potash, and magnesia, and plants which can obtain food 
from other sources than the mineral substances in the soil 
have a big advantage over their neighbours. Carniv- 
orous plants, for example, which obtain a large pro- 
portion of their food from the bodies of insects, are 
common in the wetter parts ; whilst others—e.g., all the 
heaths, cranberries, and some grasses—obtain organic 
food through the agency of fungi (mycorhiza) which 
become attached to their roots. 
The various associations of moorland-plants may be 
summarized as follows : 
1. On dry, poor soils, drainage good : 
(a) Grass-Heath, peat thin. 
(6) Calluna-Heath, peat thicker. 
(c) Vaeeinium-Moor, alpine. 
