Those obligate lime species may be found in associations on almost any calcareous rock. Watson[1164] has given us a list of species that inhabit carboniferous limestone in Britain. Wheldon and Wilson[1165] have described in West Lancashire the “grey calcareous rocks blotched with black patches of Pannarias (Placynthium nigrum) and Verrucarias, or dark gelatinous rosettes of Collemas. White and grey Lecanorae and Verrucariae spread extensively, some of them deeply pitting the surface. These more sombre or colourless species are enlivened by an intermixture of orange-yellow Physciae (Xanthoriae) and Placodii by the ochrey films of Lecanora ochracea and lemon-yellow of Lecanora xantholyta. Amongst the greenish scaly crusts of Lecanora crassa may be seen the bluish cushions of Lecidea coeruleonigricans, the whole forming an exquisite blend of tints.”

The flora recorded by Flagey[1166] on the cretaceous rocks of Algeria in the Province of Constantine does not greatly differ, some of the species being identical with those of our own country. Placodiums and Rinodinas were abundant, as also Lecanora calcarea, Acarospora percaenoides and Urceolaria actinostoma var. calcarea. Also a few Lecideae along with Verrucaria lecideoides, V. fuscella, V. calciseda and Endocarpon monstrosum. The rocks of that region are sometimes so covered with lichens that the stone is no longer visible.

Bruce Fink[1167] gives a typical community on limestone bluffs in Minnesota:

Forssell[1168] pointed out an interesting selective quality in the Gloeolichens which are associated with the gelatinous algae, Chroococcus, Gloeocapsa and Xanthocapsa. The genera containing the two former grow on siliceous rocks with the exception of Synalissa. The genera Omphalaria, Peccania, Anema, Psorotichia and Enchylium, in which Xanthocapsa is the gonidium, grow on calcareous rocks. Collemopsidium is the only Xanthocapsa associate that is silicicolous.

d. Silicicolous. There is greater variety in the mineral composition and in the nature of the surface in siliceous than in calcareous rocks; they are also more durable and give support to a large number of slow-growing forms.

Silicon enters into the composition of many different types, from the oldest volcanic to the most recent of sedimentary rocks. Some of these are of hard unyielding surface on which only a few lichens are able to attach themselves. Such a rock is instanced by Servit[1169] as occurring in Bohemia, and is known as Lydite or Lydian stone, a black flinty jasper. The association of lichens on this smooth rock was almost entirely Acarospora chlorophana and Rinodina oreina, which as we shall see occur again as a “desert” association in Nevada; these two lichens grow equally well in sun or shade, and either sheltered or exposed as regards wind and rain. Acarospora chlorophana, according to Malinowski[1170], arrives among the first on rocks newly laid bare, and forms large societies, though in time it gives place to Lecanora glaucoma (L. sordida), a common silicicolous lichen.

A difference has been pointed out by Bachmann[1171] between the lichens of acid and of basic rocks. The acid series, such as quartz- and granite-porphyry, contain 70 per cent. and more of oxide of silica; the basic—diabase and basalt—not nearly 50 per cent. He observed that Rhizocarpon geographicum was the most frequent lichen of the acid porphyry, while on basalt there were only small scattered patches. Pertusaria corallina was abundant only on granitic rocks. On the other hand Pertusaria lactea f. cinerascens, Diploschistes scruposus, D. bryophilus and Buellia leptocline preferred the basic substratum of diabase and basalt. In this case it is the chemical rather than the physical character of the rocks that affects the lichen flora, as porphyry and basalt are both close-grained, and are outwardly alike except in colouration.

Other rocks, such as granite, in which the different crystals, quartz, mica and felspar are of varying hardness, are favourite habitats as affording not only durability but a certain openness to the rhizoidal hyphae, though in Shetland, West[1172] found the granitic rocks bare owing to their too rapid weathering. In these rocks the softer basic constituents such as the mica are colonized first; the quartz remains a long time naked, though in time it also is covered. Wheldon and Wilson[1173] point out that the sandstone near to intrusive igneous rocks has become close-grained and indurated and bears Lecanora squamulosa, L. picea, Lecidea rivulosa and Rhizocarpon petraeum, which were not seen on the unaltered sandstone. It was also observed by Stahlecker[1174], that, in layered rocks, the lichen chose the surface at right angles to the layering as the hyphae thus gain an easier entrance.