967 



Ex. 6, 7), — according as one or other of the indicator-species 

 becomes the dominant element. 



j. The cliff vegetation. 



The grass-slope completes the series of the subalpine forma- 

 tions whicii entirely cover their substratum, and we now pass on 

 to consider those in wliich bare soil, soHd rock, boulders, stones 

 or gravel show up amongst the components of the vegetation. The 

 formations belonging to this category are bj' no means similar, 

 but fall into two main groups: the cliff vegetation and the 

 rocky-flat vegetation. In the former the incomplete plant- 

 covering is due to unfavourable edaphic conditions; the higher piants 

 are unable to fix themselves on the bare surface of the rocks, 

 and require loose soil, either that lodged in a crevice in the rocks 

 or that lying on a ledge. The open vegetation of the rock-flat, on 

 the other band, is due to extreme climatic conditions, especially 

 the strong wind, hence only a few particularly hardy and well- 

 adapted species are able to thrive. A feature common to both 

 these types of vegetation, and also to the grass-slope, is their oc- 

 currence in the lower zones as well as on the mountains. They 

 do not therefore quite fit into the subalpine formations with which 

 series we are at present concerned. I propose to describe the cliff 

 vegetation here amongst the subalpine formations, as has already 

 been done with the grass-slope, because these formations are more 

 frequent and better developed in the lower regions; when they do 

 occur at greater elevations, they are not essentially different, and 

 may be regarded simply as an impoverished form of the lowland 

 type. This conformity in the vegetation at different altitudes can 

 only be explained by assuming that the external conditions especi- 

 ally as regards light and the solar radiation accompanying it, are 

 much the same on any slope; the height above sea-level may vary 

 considerably, but the exposure of the place is more important than 

 its altitude. 



The rocky-flat vegetation (»Fjældmarken«) is much more an 

 alpine formalion, and rarely makes its appearance in the lower 

 regions on the Færoes. Where this formation is most at home is 

 on the flat plateaux of the mountains, and there it always occurs, 

 ranging from the lower plateaux (200—300 m. above sea-level) up 

 to the summits of the loftiest mountains (700 — 800 m.). It seems 

 therefore more natural to treat the whole of this formation as part 



