PHYSICAL GEOGRAPHY 



333 



sected with clefts, rent by frost, which are filled with- Grimmias and 

 Salijc herbacea. 



The flora of the lava- streams cannot be referred to any single 

 plant-formation, because according to the age and the progessive 

 development of the vegetation, the lava may bear on it all possible 

 kinds of plant-formations. Nevertheless in Iceland several plants 



Fig. 33. Lava-field in Nordrardal in the district of Borgarfjord (Aug. 3, 1909). 



The lava is covered to a depth of one foot with a layer of Grimmia hypnoides. 



Projecting parts of the lava are covered with crustaceous lichens. Here and 



there a tuft of Festnca ovina. Birch coppice in the background. 



(Phot. A. Hesselbo.) 



are more particularly associated with lava-streams and have there 

 found shelter in the numerous clefts and depressions, where condi- 

 tions of life are especially suitable for them. Paris qimdrifolia is 

 found only on lava-streams, and also ferns are found, in abundance 

 and often as very large specimens. Aspidinm filiv mas, A. spinnlo- 

 sum, A.lonchitis and A. phegopteris very rarely occur in other habi- 

 tats than lava-clefts; Athyrium filix femina, Woodsia ilvensis 9 Poly- 

 podium vulgare, Aspidinm dryopteris and Cystopteris fragilis are also 

 common in lava-clefts, although they are also met with fairly often 

 in other localities, between blocks of rock and in rock -clefts. 

 Milinm effiisum occurs also most frequently in lava-clefts. On lava- 



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