September 29, 1892] 



NATURE 



5«9 



members, and the upper surface of the colony is also a 

 common absorbing (and transpiring) tissue, though here 

 it is only the atmosphere gases which are taken in to the 

 interior. There is not the slightest doubt but that the 

 fungal members utilize the nitrogen of the air ; there is 

 none in the rock for them to receive ; and that the algal 

 members absorb carbonic acid gas from the air taken in, 

 and combine it with the elements of water to make car- 

 bonaceous food, for that again is not presented to them 

 from the rock ; the lichen growth cannot any more than 

 is the case with higher plants utilize the carbon of the 

 mineral carbonates in the manufacture of carbohydrate 

 or hydrocarbon food. 



In reproduction, separate spores of algae and fungi are 

 produced from the lichen, and some of these may germi- 

 nate above the parent community and unite to form afresh 

 colony upon the old, or a new colony may be produced 

 from foreign spores. In any case we find generation after 

 generation of lichens forming on the same favourable 

 spot ; but succeeding generations are partially parasitic 

 and saprophytic in nature, as is shown by the manner in 

 which their lower absorbing surfaces or prolongations act 

 on the lichen growths beneath them. By and by, when 

 perhaps a score of lichen colonies have formed one 

 above the other — the newer slowly extinguishing the life 

 of the older — a vegetable considerably higher in the 

 scale of being comes forward and caps the last lichen. 

 This is some variety of moss. Spores of mosses, carried 

 to the stone surface, germinate there in moisture, and if 

 upon lichens, the moss plantlet develops into the adult. 

 Its rhizoids pierce their way through the substance of 

 the lichens, and many get down to the decomposing 

 stone surface, while some never leave the lichen bodies. 

 The action of the moss rhizoids "on living and dead 

 lichens, also, I think, shows that that plant can be a 

 parasite and a saprophyte as well as a normal vegetable 

 feeder ; and in this respect, except in its not utilizing the 

 nitrogen of the air, resembles the later lichen growths. 



In this way, by the succession of lichens and mosses 

 (and afterwards higher plants), the essential organic ele- 

 ment of soils becomes early incorporated with the me- 

 chanically and chemically disorganizing rock. 



The dead organic matter changes in different ways : 

 first, very slowly and very indifferently, by the action of 

 air and water ; and second, rapidly, by the spreading 

 through its mass, where air has free access, of bacteria 

 and other lowly fungi which are saprophytic, but can also 

 assimilate nitrogen from the atmosphere, as is shown by 

 their increasing, and not simply maintaining the original 

 amounts of nitrogen left by their predecessors. By the 

 first method of change the organic matter becomes the 

 tougher former of humus, and humic and other related 

 acids arise from it ; by the second the mild, dry, or 

 friable humus is produced and little or no humic acid. 

 A very careful investigation shows that those bacteria 

 which have the power of removing from the dead organic 

 matter the elements of their nutrition give out by the 

 decay (which occurs rapidly) of their bodies when they 

 die the nitrogen and other elements in an active state. 

 The nitrogen of the dead bacteria forms readily nitrate of 

 lime or potash by contact with these bases. 



Now to give a short summary here. Oxidation of iron 

 is the first change perceivable in granite ; then creation 

 and multiplication of weather joints, and carbonation 

 follows ; next humus is formed by lichens, and then 

 higher plants ; following this, fungoid germs, capable of 

 assimilating aerial nitrogen, become abundant ; finally 

 all the three processes, mechanical, chemical, and organic, 

 go merrily on together and contribute all in their proper 

 shares to the formation of an ever-deepening soil, capable 

 of supporting the luxuriant life of the highest plants. The 

 humic acid which is formed by the inorganic decay of 

 humus has a certain decomposing action, but it gradually 

 changes to carbonic acid, with the action of which, in this 



NO. I 196. VOL. 46] 



connection, we have already dealt. Well, to apportion 

 the shares of the work done further. By disintegration, 

 or mechanical action, the great rough mass of the soil 

 is produced. By oxidation and carbonation, soluble 

 minerals capable of entering the plant are prepared, and 

 insoluble matters like secondary silica, pure clay, and 

 steatite, are brought into being. By the action of living 

 matter, rock decomposition is hastened, and nitrogenous 

 substance is brought into the soil. By the presence and 

 action of dead organic matter, rock decomposition is 

 also forwarded, and a field for aerial nitrogen-assimi- 

 lating germs is prepared. The table below gives a list of 

 the materials found in the youngest granite soil on which 

 nothing higher than rock-mosses are growing. 



Granite minerals in fairly fresh condition About 80 per cent. 



Clay and insoluble secondary silica ... About 3 per cent. 



Soluble silica Not determinable. 



Carbonates of potash, soda, lime, mag- 

 nesia, iron, and phosphate of lime About 2 per cent. 



Sulphates of above, except iron Not determinable. 



Sulphate of iron Merest trace. 



Peroxides of iron and manganese ... About 3 per cent. 



Humus ... ... ... ... ... 12 per cent. 



Total 

 Later on, as the soil deepens, we 



.. 100 per cent, 

 find some curious 

 changes proceeding, which I will briefly indicate. Sul- 

 phates are now produced in considerable quantities. 

 Wherever iron-containing minerals are brought into con- 

 tact with organic matter, sulphate of iron tends to form 

 as well as carbonate (humate ?), and possibly other com- 

 pounds ; and the pyrites which was slow to change at 

 the beginning now produces sulphate of iron with greater 

 rapidity. The dissolved sulphate of iron coming into 

 contact with the carbonates of the alkalies and alkaline 

 earths liberated from the felspars, hornblende, &c., as 

 already explained, causes a double decomposition. The 

 ferrous sulphate becomes a carbonate, and the carbon- 

 ates of lime, potash, soda, &c., become sulphates. The 

 iron carbonate, where exposed to air, readily oxidizes to 

 ferric oxide, the chief colouring ingredient of the soil. 



Now, in the finished soil, which, it must be remem- 

 bered, is when produced fro.n granite a loam, we have 

 the following approximate composition, as fairly typical 

 of a good granite soil such as may be found in the valley 

 of the Don in Aberdeenshire : — 



Per cen:. 

 i Pure clay and steatite about 10 



A Quartz and secondary silica ... 



( Muscovite 



Orthoclase 



Plagioclase 



Biotite 



Magnetite? 



, Hornblende 



I Hematite and limonite (ferric 

 oxides) and manganic oxide 



Pyrites 



Humus and animal organic 



\ matter, fungi, &c 



i Humic acid 



-! Soluble silicate 



(^ Ferrous sulphate^ ... 



/ Phosphates of lime, magnesia, 



[ potash, soda, &c. 



Sulphates of lime, potash, 



j soda, magnesia, &c 



Nitrates of lime, potash, soda, 



magnesia, &c 



Carbonates of lime, potash, 



soda, magnesia, &c 



Chlorides of above 

 Water and air (mechanically held) in dry 

 summer perhaps about 



Total 



> More than o'l per cent, is injurious. 



Insolubles 



Substances 



capable of 



transformation 



Injurious 

 solubles 



Non-injurious 

 solubles 



20 



4 

 30 



4 



9 



05 



2 



9 

 OS 



5 



o-S 

 03 

 05 



03 



o-i 

 Trace. 



