20 SOILS. 



formed by the plant and used in the corrosion of the rock 

 minerals. When it is remembered that this acid is comparable 

 in strength to hydrochloric and nitric acids, the energy of the 

 attack of the lichens is explained. Its progress can often be 

 traced, even beyond the visible root fibers, by a change in the 

 color of the rock ; c. g., from rust-color to brick red. 



When by the action of the lichens a certain depth of loosened 

 rock or half- formed soil has been produced, the next step is 

 usually the advent of various mosses, which gradually shade 

 out the crust-like lichens, while the erect kinds persist for some 

 time. Eventually the mosses, after having increased still 

 farther the soil layer on the rock surface, are themselves par- 

 tially or wholly displaced by the hardier species of ferns ; and 

 with these the higher flowering plants, such as the stonecrops 

 and saxifrages (the latter deriving their name from their 

 "rock-breaking" effect), the heather, and many other or 

 shallow-rooted plants, gradually take possession. The roots 

 of all plants secrete carbonic acid; and many of them, much 

 stronger vegetable acids, such as oxalic and citric. In the 

 crevices of rocks we commonly find the roots forming a dense 

 network over the surfaces, the marks of which show plainly the 

 solvent effect produced on the rock by the root secretions. 

 This is most readily observable on a polished marble surface, 

 or on feldspathic rocks. Of course the progress of soil-forma- 

 tion is very much more rapid when, as in the case of powdered 

 lava (volcanic ash) and rock debris resulting from the effects 

 of frost etc., the surface is very much increased. In tropical 

 climates, where both vegetative and chemical action is most 

 intense, it takes some of the higher plants only a few years 

 after a volcanic eruption to take possession of portions of the 

 " ash " surfaces ; thus helping to form a soil on which after a 

 few more years agricultural plants such as the vine and olive 

 yield paying returns. 



To this direct action of the higher plants is always added, 

 to a greater or less extent, that of innumerable bacteria, as 

 well as molds; whose vegetative and secretory action mater- 

 ially assists that of the roots, and the weathering process in 

 general. 



Humification. While the mechanical action of the roots and 

 the chemical effect of the acids of their root secretions are very 



