334 THORODDSEN 



streams there is a good opportunity of studying the development of 

 the different plant-formations and plant-societies because, at the be- 

 ginning, every lava stream is virgin soil, where plant-life must break 

 entirely new ground. The first establishment of vegetation is due 

 to mosses and lichens; on the 23-years-old Krakatindshraun Dr. 

 H. Jonsson found twelve species of mosses, three of lichens and 

 one alga. Two of the lichen -species, Stereocaulon alpinnm and 

 Squamaria gelida, were widely distributed on lava-domes, the mosses 

 usually occurred in clefts, but nowhere had a moss-carpet yet deve- 

 loped (H. Jonsson, 1905, pp. 55 56). The next stage is the 

 Grzmmza-heath, which occurs as a continuous covering over the 

 low-lying parts of the lava , w r hile crustaceous lichens form crusts 

 upon the protruding points; the Grimmias form the soil-layer which 

 is primarily necessary for the growth of higher plants. In this moss- 

 foundation occur several lichens and scattered specimens of flowering 

 plants from different associations. After this the development on the 

 lava-streams in the lowlands proceeds in various directions according 

 to the surface-conditions of the lava, the nature of the rock and 

 other circumstances, such as the greater or less amount of drifted 

 sand or humus which has settled upon the surface, and whether a 

 supply of water is available, etc. Consequently, in the course of 

 time a lava-stream may support either a heather-moor, a coppice- 

 wood, a "herb-flat", or grassland, or all these plant-formations may 

 be simultaneously present on the same lava-stream. Rocky- flat forma- 

 tions proper, do not occur on lava-streams, except very rarely, when 

 the lava-streams become covered with gravel brought down by moun- 

 tain streams or glacier rivers. On the plateau the vegetation on a 

 great many of the lava-streams does not go beyond the lichen-stage, 

 on others a considerable number of Grimmias are present, but al- 

 most never as a continuous covering, such as they form in many 

 places in the lowlands and especially on the peninsula of Reykjanes. 

 On the other hand the lava-streams of the plateau are frequently 

 covered with drifted sand and support a sand-vegetation which at 

 higher levels consists of Elymus arenarias and at lower levels of 

 different Salices. There are a few r instances of old, partially blocked 

 and sand-covered lava-streams in the valleys, in localities where 

 water was abundantly present, forming the substratum of swampy 

 grassland with peat. Thus, in the course of time, a lava-stream 

 may give rise to almost any formation 1 . 



For further information regarding plant-life on lava-streams see H. Jons- 



