Iv] GRASSLAND ASSOCIATIONS 123 
grassland is there the rule. In the west of Ireland, calcareous 
heath is widespread on an extensive lowland plain of Carboni- 
ferous Limestone in co. Clare (see The New Phytologist, 1908: 
259). The calcareous heaths of the present district are rather 
meagrely developed ; but examples occur at the head of Monk’s 
dale north of Miller's dale, and at the east of Longstone Edge 
north of Longstone. Other examples occur south and south- 
east of Buxton. There is rather an important difference 
between the limestone heaths of Somerset and those of the 
Peak District : those of Somerset usually occur below 850 feet 
(259 m.) and those of Derbyshire above 1000 feet (305 m.). 
Correlated with this altitudinal difference, there is a consider- 
able difference in the floristic composition of the two associations. 
Whereas the limestone heaths of Somerset are characterized 
by only some half-dozen heath-loving species (albeit these 
are often very numerous as regards individuals) and a very 
large number of lime-loving species, the limestone heaths of 
Derbyshire are characterized by a small number of lime-loving 
species (which, however, are abundant) and a large number of 
humus-loving species. A Derbyshire botanist may obtain a 
rough idea of a Somerset calcareous heath if he imagines his 
grassy dale-slopes to possess all the numerous lime-loving 
species which actually occur there, and to possess, in addition, 
numerous and well-grown plants of ling or heather (Calluna 
vulgaris), heath (Erica cinerea), gorse (Ulex spp.), and bracken 
(Pteris aquilina). It is a factor of some importance also 
that the calcareous heath of Somerset occurs on slopes which 
are much less steep than those of the limestone dales of 
the Peak District of Derbyshire. 
Graebner (1901) has also stated that Calluna may occur on 
calcareous soils, not incidentally but in quantity. A mixture 
of calcicole and calcifuge species on calcareous soils has been 
described by Géze (1908: 463—4), who also states that the 
lime is frequently leached out of the superficial layers of soil. 
It was suggested in the previous chapter that the greater 
percentage of lime on the soil of the steep calcareous slopes is 
probably due to the upper layers in such places being in process 
of slow but continuous denudation; and hence the superficial 
soil is constantly changing, and new and more highly calcareous 
layers brought into use for the plants. On the other hand, the 
