614 A. HESSELBO 



The surface of the lava-field is very dry, because the water can 

 easilv drain away through the cracks, therefore, the vegetation there 



/ / U 7 



is decidedly xerophilons. As a rule, the Rhacomitrium heath (R. 

 hypnoides) in which other Bryophyta, such as Dicranuin sco- 

 parium, Hijlocomium proliferum and Ptilidium ciliare, occur only 

 extremely scantily covers the greater part of the lava-fields, and 

 there arrives at its fullest development, so that extensive areas may 

 be found covered with foot-deep, soft, greyish carpets which hide 

 all irregularities of the surface. The importance of the moss-covering 

 for the further development of the plant-covering, partly by binding 

 the drifting sand, partly by forming soil, has been demonstrated by 

 Gronlund and Helgi Jonsson. 



The chief reason for the extensive distribution of the moss-heath 

 must undoubtedly be sought in the nature of the surface. This is, 

 as a rule, highly vesicular, and contains numerous small holes and 

 cavities, in which the plants find good conditions for taking root. 

 Therefore, many of these cavities are filled up with small moss- 

 cushions (Fig. 36), which, from thence, extend over the rock-surfaces 

 and gradually merge into one another (Helgi Jonsson, 1900, p. 83). 



The tops of the lava-cones and the protruding blocks are not 

 covered with a continuous moss-carpet, but usually with scattered 

 cushions of mosses, liverworts and lichens. The most frequent 

 species there are Rhacomitrium heterostichum, R. fascicnlare, Grimmia 

 apocarpa, G. funalis, Andrecea petrophila, Hypnum reuolntnm, Ho- 

 malothecium sericeum, Pterigynandrum filiforme, Dicranoweisia cris- 

 pula, and occasionally Gymnomitrium coralloides, G. concinnatnm, 

 Dicranum fulvellum, Rhacomitrium siideticum, Potutrichum pilosum, 

 besides which there are several other species. Thus, near Hafnar- 

 fjordur (and partly also in Budahraun), in addition to the majority 

 of the species mentioned above, there occurred Orthotrichum rupestre, 

 0. Sturmii, Grimmia patens, Leucodon sciuroides var. morensis, Isothe- 

 cium tenuinerue and FrnUania Tamarisci. Near Thingvellir, where 

 the surface of the lava is very dry, there grew upon the lava-cones 

 many crustaceous lichens, Rhacomitrium fasciculare (in abundance), 

 Grimmia apocarpa, Andrea>a petrophila, Potijtrichum pilosum and, 

 here and there, some Gymnomitrium coralloides and Dicranum ful- 

 vellum; and, where some soil had accumulated, Ceratodon purpureus 

 and Pohlia nutans grew scantily. In the depressions Rhacomitrium 

 hypnoides was gradually replaced by R. canescens, heather moor and 

 grassland, and in the deepest depressions small patches of bog and 



