226 OLAF GALLOE 
I had not time to determine the frequency-numbers of the 
crustaceous, foliaceous and fruticose lichens, but even on a super- 
ficial view, it was evident that foliaceous lichens were in the ma- 
jority, and that the vegetation was fairly dense, so that competition 
existed amongst the individuals. 
Fruticose-lichen-association, very vigorously developed, 
was observed by me on lava in another locality near Myvatn. Here 
I traversed a large tract of country, which was entirely covered by 
a thick, well-developed carpet, consisting almost exclusively of Stereo- 
caulon denudatum, which formed a very dense, pure and fine growth, 
without any intermixture of other species worth mentioning. This 
species of Sfereocaulon, as long as it is young, is able to grow on 
bare (but of course weathered) rock. Afterwards its podetia die away 
at the base, and form a peaty layer; as a consequence it gradually 
becomes an earth-lichen. But, at any rate to begin with, it can 
occur as a rock-lichen, i. e. it does not require a preceding growth 
of mosses to which to attach itself. But from this it does not follow 
that, when occurring as a carpet, it has always been the first species 
to arrive. In fact, I have observed it growing upon moss. 
Consequently, we have here to do with a form intermediate 
between an earth-lichen-association, and a rock-lichen-association. 
Another fruticose-lichen-association on lava is formed by Rama- 
lina subfarinacea, which lives immediately attached to the rock- 
substratum, and never develops into an earth-lichen. I found this 
species near Havnefjord, where it grew on the top of a mass of 
lava, mixed with some crustaceous lichens (F °/o 100), which, although 
they occurred in all the sample-areas, were evidently on the point 
of becoming overgrown, and killed by the Ramalina. 
How many species in all are found on lava, is not known, but 
a list is given above of those which occur most frequently on it. 
I am, however, inclined to believe that, practically, all the rock-lichens 
found in Iceland can grow both on lava and on basalt; partly be- 
cause the two kinds of rock are in the main of the same chemical 
composition, and partly because it appears that lichens are very 
partial to lava as a substratum. From a superficial point of view, 
the vegetation of the lava appears to be considerably richer in 
quantity, than that of the basalt; for instance, I never saw such 
immense quantities of Sfereocaulon on the latter, as on lava. But 
it is desirable that the conditions concerning masses, by the method 
