STABLE AND MIGRATORY PLANT FORMATIONS 15 
bution of the heath and other associations capable of growing 
on surfaces sterilised by acid humus (21). The further 
accumulation of acid humus to form peat, and the extent 
that the moorland and heath associations invade other surfaces 
less subject to leaching, depend mainly upon the climate 
and topography. Widespread glacial clays often have a 
thick covering of peat in Scotland, but support forests in 
other regions. 
Hilgard points out that in parts of N. America the richer 
vegetation of the lower regions only ascends to higher alti- 
tudes in limestone districts, and suggests that the comparative 
fertility is associated with the presence of lime, and that the 
poverty of the vegetation in highland districts other than 
those formed of limestone is due to the leaching of the rocks 
and the removal of the lime in solution to lower levels (18). 
Thus we must consider as the effects of physiography, not 
only the leaching of the alkaline bases from the rocks at high 
levels, but also their accumulation in the ground waters at 
low levels. The effects of this on plant association will be 
guided by the physiography and the depth and porosity of 
the soil. 
The effect of limestone on vegetation is probably much 
more complex in its physiographic relations than is usually 
assumed. Pure, soft, porous, unjointed limestones, like the 
chalk, undergo nearly equal surface solution over wide areas, 
but their success or failure in locally accumulating a leached 
soil depends on the purity of the rock, the climate, and the 
contour of the ground and the water table. 
The localised solution of hard, compact, jointed limestone 
tends to the formation of an extensive underground drainage 
system wherever such a limestone country is raised above the 
base level of erosion. The evolution of such a drainage 
system depends on the persistence of a limestone area in this 
relation for long periods under a wet climate. It is also 
probable that a soil-cap with abundant humus, and somewhat 
permeable to water, conduces to its development. An area 
consisting largely of limestone rocks without such an under- 
ground drainage system may accumulate a deep soil under 
humid conditions favourable to luxuriant vegetation. Such 
a soil, especially near the surface, may contain little lime 
