5i8 



NATURE 



[September 29, 1892 



the constant multiplying of the weather joints, which are 

 first marked out by oxidation as already indicated, and 

 afterwards made definite and widened from the most ex- 

 posed rock surface inwards by frost, one of the first steps in 

 soil formation is accomplished. As those fissures are 

 increased the uppermost portion of the rock is separated 

 by them into distinct pieces, which latter are again in 

 their turn broken up by the formation of weather joints 

 in the same way as the original. The great bulk of a 

 soil has been produced in this way. 



While the oxidation of the iron, as I have observed, 

 is very likely the first change to set in in every case, it is 

 never left for any lengthened period to promote, by itself 

 alone, the decomposition of the rock. Very soon the 

 work of carbonation is seen to be progressing alongside 

 of it, though at a considerably slower rate. The car- 

 bonic acid gas of moist air, dissolved in the penetrating 

 water, attacks the felspars, the biotite, and the horn- 

 blende. The way in which it brings about the 

 decomposition of these minerals is interesting. Certain 

 molecules succumb much more easily to the action of the 

 carbonic acid than others, and the result is that scattered 

 points of weakness from the thorough decomposition of 

 these are brought into being in different parts of the 

 •mineral, and those decomposed portions warp round 

 about the other and fresher molecules, as shown in the 

 annexed diagram, which has been constructed from what 

 I have observed in decomposed felspars. 



The clay of decomposed felspar has great plastic and 

 warping power. I have observed only 15 per cent, of 

 pure clay in a mass hold the 85 per cent, of other and 

 different constituents together in a plastic union as if the 

 whole had been pure clay. There are two or three 

 other hitherto unknown facts connected with the natural 

 -decay of felspars which I have ascertained from my re- 



Diagram of kaolinized felspar X 260. The whole ground-mass is kaolin or 

 pure clay ; the bodies scattered through this are parts of the original 

 felspar not yet decomposed. 



searches. I have noted two processes of decomposition — 

 that which occurs when the carbonic acid is in excess or 

 can obtain free access to the mineral, and that which takes 

 place when either of the opposite conditions prevails. In 

 the first case the felspar — supposing it to be orthodase — 

 has the molecules of its body which are affected com- 

 pletely broken up into clay, solid secondary or colloid 

 silica, and carbonate of potash. In the second case, 

 where for some reason a sufficient supply of carbonic 

 acid cannot get within " chemical " distance of the fel- 

 spar molecules, clay is produced as before— only more 

 slowly— but the potash of the molecule is carried off in 

 two sections, part as a carbonate, and part as a soluble 

 silicate/ From the plagioclase-{€i%^zx the same con- 

 ditions produce similar results, except that the soluble 

 sihca which would be produced here is of course in com- 

 bination with sodium. I have found the soluble siHca of 

 soils always in the form either of silicate of potassium or 

 sodium, and very frequently both of these occur mixed 

 together. 



Biotite^ by the continued action of carbonic acid, oxy- 

 gen, and water, loses magnesia (taken out as carbonate) 

 and iron (removed either as oxide or carbonate) and be- 



o^.^^f f-T^° '" '"^'^ connection my article on " The Action of Lime on Clay 

 Soils, Nature, Jan. 29, 1890. 



NO. I 196, VOL. 46] 



comes eventually the white or yellow muscovite variety 

 which undergoes no further chemical change. In biotite, 

 however, the chemical change usually takes place much 

 more uniformly through the mineral body than ever 

 happens in the case of the felspars. 



Hornblende by carbonation, oxidation, and hydration 

 yields lime as carbonate until the whole of that base is 

 taken out, a trace of magnesia as carbonate (the bulk of 

 this base is almost invariably left in the insolulale residue), 

 the chief portion perhaps of its iron as oxide and carbo- 

 nate, manganese as hydrated oxide, and any trace of 

 sodium and potassium which it may contain as carbo- 

 nates, or partially when conditions are less favourable as 

 soluble silicates. The residue left after the hornblende 

 has lost the above can generally be determined as some 

 variety of chlorite (hydrated silicate of magnesia, iron, 

 and alumina), which in the course of time by further loss 

 of iron becomes an impure serpentine, and this later on a 

 steatite or magnesia-clay, to which the greasy feel of 

 soils is due. 



The pyrites of the granite rock is slow to change, but 

 it also is eventually acted on, by water and oxygen par- 

 ticularly, the latter combining with its substance here and 

 there to form a sulphate, which has a great mission in 

 the physiology of the soil. 



I have said that the apatite occurs as an endomorph. 

 It is set free to dissolve slowly without change in ordinary 

 carbonated water when the minerals which hold its 

 microscopic needles in their substance are broken up, 

 mechanically or chemically. The magnetite and ilinenite 

 grains of the granite rock are only altered with provo- 

 king slowness. Their function, however, in the work of 

 the soil is, as far as I can see yet, of no importance. 

 Traces at least of another mineral occur very frequently 

 in granites. This is tourmaline, the history of which in 

 soils I have been investigating for the last half-a-dozen 

 years with some success. 



The chemical changes which I have been mentioning 

 begin first on the exposed surfaces of the rock and along 

 the faces of the primary joints. Then oxidation occurs 

 in streaks and bands through the rock mass, and around 

 those areas carbonation is most active. In fact oxida- 

 tion opens up the rock for further change, chemical as 

 well as mechanical. 



Frost is the principal agent of disintegration or 

 mechanical breaking up in this country, but a relatively 

 minute portion of the work is accomplished by heat and 

 cold, the friction of percolating water, changes in the 

 degree of humidity of the atmosphere, the pressure exerted 

 by roots, and so on. 



No sooner does a fraction of the surface percentage of 

 the exposed rock portion undergo chemical change than 

 a new element in the making of soil comes into play — 

 that, namely, of organic matter, first living, and then dead 

 and living. We will deal first with the living matter. 

 On the partially decomposed surface of rock, fungal and 

 algal spores (the latter of a lowly type) settle and live and 

 grow in symbiotic union as lichens. There are many 

 different kinds of rock-lichens, but the vegetative 

 physiology of all is identical. The surface of the growth 

 which lies next the stone is engaged in parts, during moist 

 weather at least, in the imbibition of water, with the ex- 

 ceedingly meagre amounts of mineral matters dissolved 

 in it from the surface of the rock. Those absorbing areas 

 of the under surface appear to be also superficial breath- 

 ing organs, for they certainly excrete carbonic acid gas, 

 which of course will join with the atmospheric carbonic 

 acid in helping the work of decomposition of mineral 

 bodies. And it appears to me — though here I am not 

 certain — that these absorbing areas are less generally 

 found over the quartz of the granite, which is not capable 

 of chemical change, than over the decomposable minerals. 

 The lower absorbing areas of the lichens are in their 

 functional relations common to internal fungal and algal 



