THE BHYOPHYTA OF ICELAM) 



()()1 



ing to a height of several hundred metres, riglil up to the plateau 

 on the niountain top, but usually divided into niinierous steps, 

 consisting of shelves or ledges of varying breadlh. The rock-face 

 is here and there inteisected by deep fissures and clefts, excavated 

 by the downward-flowing waler, which partly drains away in 



Fig. 29. Face ol" basalt rock (E. Icelanil;. Everywhere in the cre%'ices small, black, inoss- 

 cushions {Andreæa. Dicranoiueisia crispiila and Grimmia spp.). 



these channels, partly spreads over the ledges and from thence 

 flows down the rock-faces or penetrates into the crevices of the 

 rock, and then reappears once more further down. Consequently, 

 the conditions perlaining to moisture vary greatly, and therefore 

 the moss-covering also. As some examples \\'\\\ hest show the com- 

 position of the latter, the following are given: — 



1. Esja, Iow damp rock-face just above the slope: 



The Botany of Iceland. Vol. I, part. II. 39 



