612 A. HESSELBO 



from the cold, north winds, without, however, preventing the sun 

 from warming the tuff-faces. 



The Bryophyte Vegetation of the Lava-fields. 



A very great part of Iceland is covered with lava. According 

 to Thoroddsen (1914, p. 219) the post-glacial lava-fields of Iceland 

 cover an area of about 11,200 square km. or almost Vs of the entire 

 area of the island. It is, however, only a very small part of this 

 vast area which has been an object of bryological investigation. 

 Gronlund (1874, p. 136 and 1890) was the first to mention the 

 vegetation of the lava-fields, in his description of the Rhacomitriuiu 

 heath as important for the further development of the plant covering. 

 Ostenfeld (1898, p. 246) has given a description of the lava-fields 

 of the peninsula of Reykjanes without, however, entering more 

 closely into the subject of the Bryophyle vegetation. Helgi Jonsson 

 (1900, p. 89) has given a very detailed account of the vegetation of 

 Budahraun and Eldhraun in West Iceland, also with regard to the 

 Bryophyta, and the same author, in his description of the vegetation 

 of South Iceland, mentions the Bryophyte vegetation of several lava- 

 fields, especially modern ones. Lastly, A. Wegener in 1912 made 

 a few collections on the high-lying Odadahraun (600 1200 metres 

 above sea-level). 



Besides a small part of Myvatnshraun, around the farm Rey- 

 kjahlid, which was very superficially investigated owing to the 

 inclemency of the weather, my own investigations include only the 

 lava-fields o SW. Iceland, especially Hafnarfjardarrhraun, Thingvalla- 

 hraun and Svinahraun around Kolvidarhol. 



The lava-field is no single formation, but, on the contrary, in- 

 cludes a great number of different formations, ranging from the dry 

 rock-flats, and the Rhacomitrium heath, through the heather moor 

 and grassland to the bog formations in the deepest hollows, and 

 the associations of damp rocks in clefts. When it is nevertheless 

 described here in a separate section, as by Helgi Jonsson, the 

 reason is that the vegetation of the lava-fields of the lowlands every- 

 where presents so many common features and peculiarities that we 

 may be justified in describing the Bryophyte vegetation occurring 

 there. 



According to the nature of the surface, the lava-fields may be 

 classified into two groups, viz. block-lava (Icelandic Apalhrann) and 

 sheet-lava (Icelandic Helluhrann). The former consists of blocks of 



