LICHENOLOGY OF ICELAND 227 



of weight, and the number of the species, may be investigated 

 more thoroughly. 



c. Tuff. 



On the tuff-deposits of Iceland, lichens occur very sparsely. 

 The tuff consists of rather loosely-connected ash-particles, and is 

 naturally stratified like other aeolian sedimentary deposits. Its che- 

 mical composition comes very near to that of the basalt and the 

 lava, but its physical conditions, as a plant-substratum, differ very 

 essentially, since, in the first place, its porosity causes all the water 

 which falls upon it, to be absorbed and retained, as in a piece of 

 blotting-paper, so that the lichens are deprived of this water; and 

 in the second place it has, in all probability, quite different thermal 

 qualities, inasmuch as it no doubt gets heated far more slowly than 

 the two other kinds of rock. 



Taken as a whole, it may be said that tuff is a very unfavour- 

 able lichen-substratum. But then I must acknowledge that I saw 

 it only in a few places, partly as a shore-rock, where it was quite 

 devoid of lichens, since both Verrucaria- and Ca lop laca- vegeta- 

 tion were totally absent; and partly in the interior of the country, 

 where, in a few places, I saw old crater-cones (extinct) consisting 

 of tuff, where the vegetation was so scanty that everything but 

 lichens was wanting, consequently both mosses and phanerogams, 

 whilst the lichen-vegetation was restricted to a few specimens, which, 

 if the frequency- number had been determined, would hardly have 

 amounted to one individual per 1000 sample-areas (a 2 dm. 2 ). 



Not far from the farm Ljosavatn, between Hals and EinarstaSir 

 (North Iceland) I found specimens perhaps 50 in all of a 

 sterile, undeterminable, crustaceous lichen. On tuff in Rey5arfjor5ur 

 (East Iceland) I found a few specimens of 



Lecanora Hageni, 



calcarea, 

 Arthonia ruderalis. 



Such paucity of lichens as this on porous rock, is known 

 almost nowhere else but in Denmark in the case of the chalk. It is 

 possible that the cause of this is the same in both cases, viz., the 

 unfavourable conditions with regard to moisture. I can express no 

 opinion as to whether the tuff occurs anywhere on the island, 

 under conditions which permit it to bear lichens, but, in the litera- 



15* 



