Inspection of the greenish rocks [see image and image description of yesterday] revealed that they are, to a great extent, covert by crustose lichens of the genus Rhizocarpon. In fact, during my hike I got the impression that all rocky mountain slopes in the Grimsel area are covered with these lichens, from the tree line up to ice and snow.
Rhizocarpon geographicum is one of the most widely distributed species of crustose lichens growing on exposed acid or neutral rocks [meaning: it is found on granite but not on limestone]. Morphologically, the lichen thallus [the entire lichen colony in my photograph] consists of discreet areolae [the small yellow-green areas within the black meshwork] that contain the cells of the alga Trebouxia, located on a fungal medulla which is attached to the rock. The black zones are algal-free and contain only the fungus. Usually, numerous thalli of Rhizocarpon geographicum grow adjacent to each other, leading to the appearance of continents on a map [hence the name of the lichen].