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 Faeroes, 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 inqhes 

 in diameter on the average, which have fallen from the dis- 

 integrating cliffs above. Such stretches of weathered debris 

 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 



