GEOGRAPHICAL DISTRIBUTION. 



103 



to calcareous rocks, as Urceolaria calcarea, Gyalecta cupic- 

 laris, Lecidea calcarea and imw.ersa ; a few are chiefly 

 found on arenaceous rocks, as Lecanora atra, and the saxi- 

 colous forms of Lecidea aurantiaca ; many are common on 

 the granitoid series, including therein micaceous, gneissic, 

 granitic, and quartzose rocks, as Lecanora tartarea and ven- 

 tosa, Parmelia conspersa and FaMunensis, and many Leci- 

 dea ; and Lecidea geograjoldca is frequent on the purest and 

 smoothest quartz. The student who gives attention to this 

 subject will soon observe that, while Lichens are abundant on 

 sandstone and granitic rocks in Scotland, — on the former 

 in lowland, on the latter in alpine regions, — they are com- 

 paratively rare on the basalt series. The nature of the ha- 

 bitat appears to exercise an influence distinct from that of 

 climate ; for we have found granitoid boulders lying at a 

 slight elevation above the sea, in trap and sandstone districts, 

 covered with subalpine or alpine Lichens. Schimper de- 

 scribes the sandstone of the Yosges, which produces a sandy 

 soil, as having a richer lichenose vegetation than that of Al- 

 sace, which gives rise to a clayey marl. On mountains it is 

 frequently easy to determine that Lichens, especially crusta- 

 ceous species, exist in greater abundance and perfection in 

 situations exposed to the north and west. Of Lichens which 



