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v] ASSOCIATIONS OF ROCKS AND SCREES 137 
L. Melica nutans. Local L. C. ornithopoda. Local 
M. uniflora C. pallescens. Local 
Festuca ovina Tamus communis 
Poa nemoralis. Local L. Convallaria majalis. Local 
Carex pulicaris. Local 
Limestone pavements, which are so characteristic of the 
limestone plateau of the mid-Pennines (see Smith and Rankin, 
1903: 167) and of the lowland limestone plain of Co. Clare 
(see New Phytologist, 1908: 258) scarcely occur in the Peak 
District of Derbyshire. 
Ostenfeld (1908: 972), in his account of the vegetation of 
the cliffs of the Faerdées, states that the water-content of the 
soil, “before all others is the factor which has the greatest 
influence, and is the first and most important condition in 
differentiating between plant associations with the same geo- 
graphical and topographical position”: this remark is doubtless 
true when the rocks and soils in question are of a similar 
chemical composition; but such a classification of the plant 
associations of a district which, like the Peak District, consists 
on the one hand of sandstone rocks and siliceous soils and of 
limestone rocks and highly calcareous soils on the other, would 
give a very queer and a most unnatural result. Water-content 
alone fails to supply a primary differentiating factor of the plant 
associations in a district like this where sandstone rocks are 
sharply contrasted with limestone rocks. The only primary 
factor giving a natural classification of the plant associations of 
the terrestrial soils of this district is one based on the presence 
as contrasted with the comparative absence of lime in the soil. 
Secondarily, or when applied either to the siliceous or to the 
calcareous soils alone, water-content becomes a decisive eco- 
logical factor; but even this is complicated by the acidic 
humus-content of many of the siliceous soils. 
Limestone Screes 
The screes consist of angular pieces of rock, a few inches 
in diameter on the average, which have fallen from the dis- 
integrating cliffs above. Such stretches of weathered débris 
are of common occurrence on the slopes of the hills in the 
limestone area. The screes of this district, however, are not 
specially well developed; and in no cases are they difficult or 
dangerous to traverse. The vegetation of the limestone screes 
