THE BRYOPHYTA OF ICELAND 



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lava throwii togelher into irregular heaps, while sheet-lava consisls 

 of large, irregularly broken surfaces, which are formed by a process 

 of slow solidification, and are furrowed by innumerable clefts and 

 fissures, both narrow and broad. Here vesicular hollows are also 

 found, produced by the moisture of the substratum which has formed 

 large steam-vesicles in the molten lava, and after solidification the 



Fig. 35. Lava-field in the dislrict of Borgarljordur. The htva is coveret! to a depth of 

 one foot with a laj'cr of Rhacomitriuni hypnoides. Projecling parts of the hiva are covered 

 with cnistaceous lichens. Here and there a tuft of Fesliicn ni>ina. Birch coppice in 



tlie backaround. 



roof of the vesicle has collapsed, and has formed a platform on 

 the floor of the cavity. Such vesicles in the lava — which are 

 characterized by very luxuriant vegetation, since they afTord excellent 

 shelter for the piants, and are inaccessible to sheep — may differ 

 rather greatly in depth, according lo the thickness of the lava-sheet. 

 In Budahraun, according to Jonsson, their depth averages from 

 2 — 6 metres, but is usually somewhat above 2 metres, and in 

 Hafnarfjardarrhraun the depth was also usually some 2 metres. The 

 lava-fields are often furrowed by deep clefts or cracks, frequently 

 of considerable length and with perpendicular sides. This is, for 

 instance, the case in Thingvallahraun where the best-known cleft, 

 Almannagja, has a length of almost 15 kilometres. 



