226 OLAF GALLOE 



I had not time to determine the frequency-numbers of the 

 crustaceous, foliaceous and fruticose lichens, hut 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 demidalnm, which formed a very dense, pure and fine growth, 

 without any intermixture of other species worth mentioning. This 

 species of Stereocaulon, 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- 

 linn 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 Stereocaulon on the latter, as on lava. But 

 it is desirable that the conditions concerning masses, by the method 



