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 æolian 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 Caloplaca-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 (à 2 dm.?). 
Not far from the farm Ljösavatn, between Häls and Einarstadir 
(North Iceland) I found specimens — perhaps 50 in all — of a 
sterile, undeterminable, crustaceous lichen. On tuff in Reydarfjöröur 
(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- 
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