BBITAIN. 



253 



moist peat these are Junaus squarrosus, Nardtis strida, Molinia depawperata, or 

 Erica tetralix; in rocky places the commumties consist of Pteris aguilina and 

 Vaccinium myrtilliis. 



Crampton (1911 : 25) has made a detailed investigation of the communities 

 of the seres of the alpine, moorland, lowland, and coastal vegetation of Caith- 

 ness. The following synopsis will serve to indicate the general lines of 

 development: 



I. Alpine and subalpine plant formations. 



1. Plant formation of the frost debris of the alpine plateaus. 



(1) " Rhacomitriumr<sa,rpet" closed association. 



(2) "CoHMno-lichen-mat" semiopen association. 



(3) "Arctostaphyliis-ina,t" association. 



2. Plant formation of the alpine crags. 



(1) Rock-siuface associations. 



Bryophyte associations of damp crags. 

 Lichen-bryophyte associations of dry crags. 



(2) Rock-crevice associations. 



3. Plant formations of the alpine scree-slopes. 



Conglomerate-block scree subformation: 



(1) Rock-surface associations of conglomerate blocks. 



(2) Crevice associations of conglomerate block screes. 



(3) Vacdnium and Alchemilla associations of marginal loose gravel. 



(4) " CaUuna-ux&t" and "Arctostaphylus-m.a,t" associations of fixed gravel. 

 Quartzite-scree subformation: 



(1) Vaccinium association marginal to moving screes. 



(2) Calluna-Erica open heath of more stable areas of scree material. 



4. Plant formations of subalpine crags of glacial origin. 



(1) Rock-surface, lichen-bryophyte associations of calcareous flagstone crags, 



(2) Rock-surface, lichen-bryophsrte associations of "leached" sandstone crags. 



(3) Rock-crevice associations and ledge-debris (chomophjrte) associations, progres- 



sive to birch-hazel scrub and grass heath. 



5. Plant formation of the steeper subalpine drift-slopes. 



Ca2{u72a-heath and grass associations, probably normally progressive to birch-scrub. 

 I. The moorland plant formation. 



General or 



climatic 



retrogression 



Topographic 

 retrogression 



Sphagnum dominant (the recessive association at present). 

 1 ^ 



Eriophorum dominant 



Scirpus dominant 



, i 



Calluna dominant 



I 



Calluna, Vaccinium 



Empetrum , etc. 



\ 



Stream Belt and Sub' 

 alpine slopes 



•• Rhacomitrium d ominant 

 Erosion into hags by wind 

 Empetrum and Salix dominant 

 Coastal belt 



Calluna and Juncus sgoarrosus 



I 



Erosion into hags by wind 



Fig. 13. — Serai relations of moorland, Caithness, Scotland. After Crampton. 



III. Plant formations zonal to the drainage system. 



1. Plant formations of the lake margins. 



2. The fenland plant formation. 



3. The plant formations of the stream belt. 



