326 THORODDSEN 



plateau patches of Anthelia uiualis occur. The rocky-flat-formation 

 appears to be an original, late Glacial formation from which a great 

 many distinct formations have developed, the different species having 

 become associated according to their conditions of life. The outer 

 limits of the rocky flat are the Grimmia-heath and the "herb-flat," 

 but transitional stages to heather-moor and grassland often occur. 

 As sub-divisions or nearly related formations the following may be 

 mentioned: - -gravelly flats (melar), stone-covered ridges (holt); fallen 

 blocks and debris upon mountain slopes (urd, pi. urdir), steep cliffs 

 (hamrar), gravelly river-plains and river-terraces (eyrar), sandy tracts 

 of various kinds, clayey flats and lava-streams. 



Gravelly flats (melar) also occupy large areas in the lowlands; 

 the soil-conditions differ somewhat, but generally the gravel is mixed 

 with clay and then the surface often cracks into polygonal cakes 

 and forms a "rudemark" (p. 257). These "rudemarks" greatly influence 

 plant-distribution, as the plants generally resort to the gravel bands 

 between the cakes, where they find shelter and protection. Some- 

 times gravelly flats are so poor in plant-life that they appear quite 

 bare and naked; sometimes they are so densely covered as almost 

 to form a "herb-flat." The most common plants on gravelly flats 

 in the lowlands are Cerastium alpinum, Arabis petrcea, Draba hirta, 

 Silene acaulis, S. maritima, Armeria maritima, Salix herbacea, Sagina 

 nodosa, Spergula arvensis, Arenaria ciliata, Alsine uerna, Thymus 

 serpyllum, Dryas octopetala, Papaver nudicaule, Oxyria digyna, Rumex 

 acetosella, Trisetum subspicatum, Poa glauca, Festnca ovina, Agrostis 

 alba, Luzula multiflora, L. spicata, etc. Naturally all the above- 

 mentioned species do not occur together; in some places a great 

 many of them may occur, while in other places a very few, perhaps 

 only three or four, may be found. The vegetation is also somewhat 

 dependent upon neighbouring plant-formations. Usually mosses or 

 lichens are very sparsely present upon these gravelly flats in the 

 lowlands; only here and there small Grimmia-cushions occur. 



On stone- covered ridges (holt) there is usually a greater 

 variety as regards soil and situation than on the gravelly flats, and 

 the vegetation there is sometimes fairly luxuriant and conspicuous 

 especially in early summer when Silene acaulis, Dryas octopetala and 

 Thymus serphgllum are in bloom; these are very common there, 

 and also the majority of the plants of gravelly flats. Moreover, the 

 following are noteworthy: Alchimilla alpiua, Saxifraga ccespitosa 

 and S. oppositi folia, Viscaria alpina, Empetnim uigrum, Sedum acre 



