THE BRYOPHYTA OF ICELAND 



617 



Very frequently a large form of Hypnnm callichroum, with regularly 

 two-rowed ramification, is the most abundant constituent of the 

 vegetation, especially in somewhat damp and shady clefts. Eurhyn- 

 chium pilifernm is often met with very abundantly, especially where 



Fig. 38. Lava-cleft near Thingvellir. At the bottom there is a very luxuriant vegetation 

 of ferns, Geranium silvuticum, etc., and in the crevices large moss-cushions occur everywhere. 



the bottom is grass-covered, and several other southern forms, for 

 instance Eurhynchium Slockesii and Isothecium myiirum, likewise 

 have a preference for the sheltered lava-clefts. 



In the very deep lava-clefts the conditions are somewhat dif- 

 ferent from the aforesaid. Where the cleft is broad enough to allow 

 the sun to shine on the bottom during a part of the day, a luxuriant 

 covering of ferns, flowering plants or mosses is found (Fig. 38); but 

 where the cleft is so narrow that the light is strongly subdued, the 



The Botany of Iceland. Vol. I, part 11. 40 



