THE LICHENS OF PERTHSHIRE 7 
in Glen Lednoch to the south of Ben Chonzie, in Glen Shee, and 
elsewhere. Diorite, felsite, basalt, and other obtrusive igneous 
rocks also occur = various places. In the valley of the Forth, 
above Stirling, there is a staal ieee area of alluvium, as also 
in Strath Dochart, above Killin. Almost all parts of the county 
show evidences of extensive glaciation, the rock surfaces in the 
valleys, and even high up the mountain-sides, being rounded and 
floors of the valleys are often covered with boulder clay and large 
deposits of gravel, either morainic or brought down by streams. 
Travelled boulders occur almost everywhere. Many of these, 
composed of mica-schist from the Highlands, have been carried 
upon them lichens characteristic of the locality from which they 
came, and where the rock occurs im situ. It is of course not to 
found upon the stones the special conditions suited to their 
growth. 
Puant ASSOCIATIONS. 
Lichens occupy a very subordinate position in the majority 
of the leona defined by plant geographers. Their presence 
is usually determined by conditions that have not been of primary 
importance in influencing the succession of the plant community, 
and pooeeney they play an insignificant part in the evolution 
the dom ee ng groups of higher plants. The few formations 
, and in combination sa other lowly cryptogams—alge, 
mosses, and hepatics—form vital links in the chain of succession, 
and are sometimes the dominant species. The Dune Formation 
f sea coasts (not represented in Perthshire) provides an excellent 
illustration. A parallel and even more interesting example is 
afforded by the Arctic-Alpine Formation, in which at least one 
association is aw dominated by lower cryptogams. This 
will be referred to la 
In the more stable ee communities, the lower cryptogamic 
vegetation is almost entirely accessory and dependent, and so we 
very diverse plant formations. In endeavouring to broadly indi- 
cate the distribution of the Scns through so large an area, we 
have realised that this cannot be done effectively without the 
help of a larger number of workers. The difficulty of determining 
lichens in the field makes it a matter of impossibility to give 
complete lists, and we have obtained but - tle help —. pub- 
lished works by plant ee We have adopted the group- 
ing of formations proposed by W. G. Smith, ‘dividing the county 
