PHYSICAL GEOGRAPHY 303 



on the plateau. Some plant-formations such as birch-coppices and 

 heather-moors, do not, however, extend to the highest levels, and 

 many species disappear on approaching the snow-line; on the other 

 hand, others, as mentioned above, are distributed in great abund- 

 ance all over the island, from the sea-level to the snow-line. 



The Icelandic climate affords good conditions of life for mosses, 

 therefore Iceland has a luxuriant moss-vegetation which is not only 

 shown in the great extent of the Grimmia-heaths, but proofs of it 

 are seen in various other ways in nature. On the interior plateau 

 in the most barren localities small green oases are often seen, con- 

 sisting exclusively of mosses; on the abrupt faces of rocks they form 

 bright green cushions around springs; and at the numerous waterfalls 

 of Iceland there is a luxuriant moss- vegetation, rich in forms; blocks 

 of rock and steep rock-faces are often covered with mosses, as also 

 the numerous cracks in the lava-streams; and the damp rock-clefts 

 are often rich in different species. Under various conditions of na- 

 ture, various species of mosses are the dominant ones, and form 

 various characteristic societies. 



A considerable number of lowland species have an upper limit 

 on mountains and plateau, but this has not as yet been thoroughly 

 investigated. On the other hand, very few highland plants have a 

 lower limit; the majority of the plants which grow 7 near the snow- 

 line thrive just as well in the neighbourhood of the sea. In many 

 districts, and in some places on the northern peninsulas towards the 

 North Atlantic, plant-associations with well-marked plateau-characters 

 are seen in the vicinity of the sea ; this is especially the case with 

 associations of Salix herbacea, Sibbaldia procumbens and Gnaphalium 

 supinum, which are otherwise peculiar to the plateaus. Of the com- 

 monly distributed species probably very few occur exclusively on 

 the plateau; of such plants only Ranunculus glacialis is known; it 

 grows in many localities near the snow-line and has doubtless only 

 rarely been found below 300 metres. Pedicularis flammed has a 

 similar distribution, but in some places it grows perhaps further 

 downwards. Otherwise there are only a few rarer species which 

 have been found only on mountains and plateau, but it is possible 

 that on a closer investigation these may also be found at a lower 

 level. Of these may be mentioned Carex pedata, Poa laxa, Cata- 

 brosa algida, Sagina nivalis, Draba alpina, Ranunculus pygm&us, 

 Diapensia lapponica, Campanula uniftora, Antennaria alpina and 

 Erigeron uniflorus. Of common plateau-plants which also occur in 



