LICHKNOLOGY OF ICELAND 189 



favourable to the lichens. Whether the absolute amount of lichens 

 (in weight per unit of area) is really greater in the high land than 

 in the low land is perhaps doubtful. But there is nothing to prevent 

 the assumption that the amount, in proportion to the mosses, is 

 greatest at a high altitude, not because the climate of mountain- 

 heights is favourable to lichens, and is more agreeable to them than 

 the climate of the low land, but because it is more inimical to the 

 mosses than to the lichens, and thereby causes the latter to grow 

 in apparently greater luxuriance. It is absolutely necessary to warn 

 against a too strong belief in one's first-hand and direct impression 

 as regards this matter; only actual measurements can give real 

 information. 



As an instance of a decided (irimnua-healh I shall describe 

 more fully a stretch of land near Havnefjord (South-west Iceland). 



The substratum is an older lava-field consisting of highly vesi- 

 cular and porous post-glacial lava, the surface of which varies greatly, 

 in that there occur level plains of small-sized lava-debris, strewn 

 here and there with a little soil, large blocks of rock of varied 

 appearance, vertical faces of rock, and caves, all mixed together in 

 great confusion. 



The Grimmia- carpet extends chiefly over the level plains covered 

 with lava-debris. An enumeration of the characteristic plants gave 

 the following results : Grimmia occurred in all the sample-areas 

 (F % 100), crustaceous lichens (F % 65), fruticose lichens 

 (F /o 15), bare ground (F /o 10), grass (F % 65), Galium (F % 60), 

 and some Silene acaulis. 



The numbers may lead us to believe that crustaceous lichens 

 highly dominate the plant-physiognomy of the carpet; this is, how- 

 ever, not the case. 



The following species of lichens have been found : 



Cetraria aculeata, fruticose. Pertusaria corallina, crustaceous. 



Cladonia coccifera, Sterile crustaceous lichens. 



But several other species may occur. If we enumerate all the 

 species which have hitherto been recorded, we get the following: 



Alectoria ochroleuca v. cincinnata, podetia-wandering fruticose lichen. 

 Cladonia rangiferina, 

 Thamnolia vermicularis, 

 Cladonia uncialis, 



