c 14 A. HESSELBO 



The siirface of the lava-field is veiy dry, because Ihe water can 

 easily drain away through the cracks, Iherefore, the vegetation there 

 is dccidedly xerophilous. As a rule, the Rhacomitrium heath (R. 

 hijpnoides) — in which other Bryophyta, sucli as Dicranum sco- 

 pariiiin , Hijlocomiiun proliferuin and Plilidium ciliare, occur only 

 extremely scantily — covers the greater part of the lava-helds, and 

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

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

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

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

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

 Gronlund and Helgi Jonsson. 



The chief reason for the extensive distril)ulion 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 piants find good conditions for taking root. 

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

 cushions (Fig. 36), wdiich, 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 frequcnt 

 species there are Bhacomitrium heterostichiim, K. fasciculare, Grininiia 

 apocarpa, G. fiinalis, Andreæa petrophila, Hypniiin revohitum, Ho- 

 malotheciuin sericeuin, Pterigynandrum fdiforme, Dicranoweisia cris- 

 pula, and occasionally Gijmnomitriiun coralloides, G. concinnatiim, 

 Dicranum fulvellum, Rhacomitrium siideticnm, Polytricbum pilosum, 

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

 fjordur (and parlly also in Biidahraun), in addition to the majority 

 of the species mentioned above, there occurred Orthotrichiim rupestre, 

 O. Stiirmii, Grimmia patens, Leucodon sciuroides var. morensis, Isothe- 

 cium tenuinerve and Frullania Tamarisci. Near Thingvellir, where 

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

 many crustaceous lichens, Rhacomitrium fasciculare (in abundance), 

 Grimmia apocarpa, Andreæa petrophda, Polytrichum pilosum and, 

 hcre and there, some Gymnomitrium coralloides and Dicranum ful- 

 vellum; and, where some soil had accumulaled, 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 i)atches of bog and 



