600 A. HESSELBO 



of ' Plagiothetiiim depressum, the ground between the blocks was 

 covered with a thick, dark-green carpet of Hypnum uncinatum, 

 Hylocomium squarrosum, Brachythecinm salebrosum, : 'Eurhynchium 

 Stockesii, Plagiothechim Roeseanum, : Thuidium tamariscinum , T. 

 abietinum , ] Mnium iindulatiim , Ditrichum flexicanle, Dichodontium 

 pettueidum, ' :: Lophocolea cuspidata, *L. minor and Lophozia quinque- 

 dentata. On the soil-covered grassy ledges and slopes, between the 

 stones, grew Brachythecinm albicans, B. salebrosum (in masses), Cli- 

 macium dendroides (in abundance), Camptothecium lutescens, Hyloco- 

 mium proliferum, H. parietinum (sparingly) and Mnium affine (in 

 abundance). 



All the species marked * are southern forms the majority of 

 which have only been found in South Iceland. On the whole, the 

 Urd affords favourable habitats for many species which do not 

 thrive in more exposed localities. 



On a dry Urd consisting of large blocks of basalt, near Holt, 

 in South Iceland, the following species were growing: Rhacomi- 

 trium fascicalare , R- heterostichum , R. hypnoides, Grimmia Doniana, 

 Schistidium apocarpum, Andrewa petrophila, Antitrichia curtipendula, 

 Hypnum uncinatum, Hylocomium loreum, H. proliferum, H. parieti- 

 num and Frullania Tamarisci. 



Almost the same species are found on larger detached blocks. 

 On blocks of lava round about Reykjavik there grew particularly 

 Rhacomitrium heterostichum, R. fasciculare, Andrecea petrophila, Schi- 

 stidium apocarpum, Grimmia Doniana and Dicranoweisia crispula. 



On larger stones in the home-field (Tunet) near Valla nes 

 (East Iceland) there grew Pterigynandrum filiforme, Hypnum revolii- 

 /H/n, Tortula ruralis and Encalypta rhabdocarpa. Some large blocks 

 near Hafursholt in South Iceland were quite overgrown with 

 Grimmia alpestris. On some large blocks of basalt near Selj a land 

 there grew Grimmia Doniana, Rhacomitrium fasciculare, R. hypnoides, 

 R. sudeticum, Dicranoweisia crispula and Gymnomitrium coralloides. 



The Vertical Rock -belts. Owing to the division into layers 

 peculiar to the basalt, the mountain sides consist of numerous 

 vertical scarps of varying height, alternating with slopes formed by 

 fallen blocks and debris. As a rule, the first 100 200 metres of the 

 mountain form a slope which is only slightly inclined at the bottom 

 and becomes steeper higher up, and where only here and there 

 small portions of the primitive rock protrude. This slope is limited 

 by a precipitous rock-face of very varying height, sometimes ris- 



