600 A. HESSELBO 



of *Plagiothecinm depressum, the groiind between the blocks was 

 covered whh a Ihick, dark-grcen carpet of Hypnuin iincinafum, 

 Hyloconiiuin Sfjuarrosum , Brachijtheciiiin salehrosiim, ' Eiirhijuchiiim 

 Stockesii, Plagiotheciiim Roeseanum, ' Thiiidiiim tamariscinnm , T. 

 abietimim , ■'Mniiim iindulatum, Ditrichiim flexicaiile, Dichodontiiim 

 pellncidiim. "Lophocolea ciispidafa, *L. minor and Lophozia quinque- 

 dentala. On the soil-covered grassy ledges and slopes, belween the 

 stones, grew Bracliytheciiim albicans, B. salehrosum (in masses), Cli- 

 maciiim dendroides (in abundance), Camptotheciiim lutescens, Hyloco- 

 miiim proliferiim, H. parietiniim (sparingly) and Mniiim af fine (in 

 abundance). 



All the species marked ■•' are southein forms the majorily of 

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

 Urd afYords favourable habitats for many species which do not 

 thrive in more exposed localilies. 



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

 in South Iceland, the following species ^Yere growing: — Rhacomi- 

 triiim fasciculare, R. heterostichum , R. hypnoides, Grimmia Doniana, 

 Schistidinm apocarpiini, Andreæa petrophila, Antitrichia curtipendula, 

 Hypnum uncinatiim, Hylocomiiim loreum, H. proliferiim, H.parieti- 

 num and Friillania Tamarisci. 



Almost the same species are found on larger detached blocks. 

 On blocks of lava round about Reykjavik there grew particularly 

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

 stidium apocarpiim, Grimmia Doniana and Dicranoweisia crisj)ula. 



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

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

 tum, Tortilla 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 al and 

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

 R. sudeticum, Dicranoweisia crispula and Gymnomitriiim 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- 



