SOILS. 



265 



easily explained as the matter seems to be 

 assumed by the authors just quoted ; aud that 

 difficulty attends the explanation of their origin 

 may be inferred from the fact of most geologists 

 having hitherto paid little attention to the re- 

 lations of the loose materials composing the sur- 

 face of the globe. They are well acquainted, 

 and coincide in opinion, with the relations of 

 the indurated rocks which form the crust of 

 the earth, but are far from being agreed as to 

 the causes which have placed the enormous 

 masses of incoherent matter, met with in every 

 quarter of the globe, in their present positions. 

 These masses of clay, sand, and gravel, bear no 

 fixed relation to one another, like the indurated 

 rooks, and, therefore, have not been placed by 

 the operation of any law of order, but simply 

 that of gravity; and it is this want of order in 

 their position which bafles the ability of the 

 geologist to ascribe the origin correctly." 



The highly respectable authority last quoted 

 further remarks : " Notwithstanding the possi- 

 bility of the formation of mould upon the sur- 

 face of hard rocks by means of atmospheric in- 

 fluences, there cannot, I think, a doubt exist 

 that by far the largest proportion of the agricul- 

 tural soil is based upon the incoherent, and not 

 on the indurated rooks." The term rock, it 

 should be observed, is used here as applied by 

 geologists, " not only to the hard substance to 

 which this name is commonly given, but also to 

 those various sands, gravels, shales, marls, or 

 clays which form beds, strata, or masses ;" and 

 the conclusion arrived at by Mr Stephens is, 

 " that the agricultural soils of the low part of a 

 country are generally not derived from the hard 

 rocks upon which they may happen to be placed, 

 but have been brought to their present position 

 from a distance by means of water." After stat- 

 ing various cases which have come under his 

 own observation, he proceeds : " It is not denied 

 that the chemical action of the air, and the phy- 

 sical force of rain, frost, and wind, produce 

 visible effects upon the most indurated rocks; 

 but of course they must have much greater 

 effects upon incoherent ones. Combined in their 

 action, they could only originate a mere coating 

 of soE over the surface of indurated rock, if the 

 rock were situated within the region of phano- 

 gamous vegetation, because it would then be 

 constantly covered with plants. But the plants 

 in their turn would protect the rocks against the 

 action of external agencies; and although they 

 could not entirely prevent, they would at least 

 retard the accumulation of soil beyond what the 

 supply of vegetation required. Some other agent, 

 therefore, more powerful than the ordinary atmo- 

 spherical elements, must be brought to bear on 

 indurated rocks, before a satisfactory solution of 

 the formation of soils can be accepted. That 

 other agent is water; but the moment we assent 

 to the agency of water being able, by its abrasive 

 power and buoyant property, when in motion, 

 to transport the abraded parts of rocks to a 

 distance, and let them fall in coming in contact 

 with some opposing bai-rier, that moment we 

 must abandon the idea of the soils formed from 

 abradedmatter being derived from the indurated 

 rock upon which they rest. Mr Buist draws these 



conclusions, after describing the relative positions 

 of deposits to the rocks upon which they I'est, 

 in a large and important district of Perthshire, 

 where he says "that the alluvial matters of these 

 districts, in general, belong to periods much 

 more remote than those originally assigned to 

 them, and came into existence under circum- 

 stances pj-odigiously different from those which 

 presently obtain ; that the present causes — that 

 is, the action of our modern rivers, brooks, aud 

 torrents, and of the air and water on the surface 

 now exposed to them — have had but little share 

 in modifying our alluvial formations, or bringing 

 them into their present form." Professor John- 

 ston, in " Lectures on Agricultui-al Chemistry 

 and Geology," sums up this matter in the fol- 

 lowing words : " Thus the general conclusion is 

 fairly drawn, that the earthy matter of all soils 

 has been produced by the gradual decay, degra- 

 dation, or crumbling down of previously existing 

 rocks. It is evident, therefore, that whenever 

 a soil rests immediately upon the rock from 

 which it has been derived, it may be expected 

 to partake more or less of the composition and 

 character of that rock ; and that where the soil 

 forms only the surface-layer of a considerable 

 depth of transported materials, it may have no 

 relation whatever, either in mineralogioal cha- 

 racter or in chemical constitution, to the imme- 

 diately subjacent rocks. The soils of Great 

 Britain are divisible into two such classes. In 

 some countries an acquaintance with the pre- 

 vailing rock of the district enables us to predict 

 the general character and quality of the soil ; in 

 others — and nearly all our coal-fields are in this 

 case — the general character and capabilities of 

 the soil have no relation whatever to the rocks 

 on which the loose materials immediately rest." 

 Sir H. Davy, in accounting for the transition 

 from rock to soil, remarks that the operation 

 may be easily conceived by referring to the in- 

 stance of soft granite, or porcelain granite. This 

 substance consists of three ingredients — quartz, 

 feldspar, and mica. The quartz is almost pure 

 silicious earth in a crystalline form ; the feld- 

 spar and mica are very compounded substances: 

 both contain silica, alumina, and oxide of iron. 

 In the feldspar there is usually lime and potass ; 

 in the mica, lime and magnesia. When a granite 

 rock of this kind has been long exposed to the 

 influence of air and water, the lime and potass 

 contained in its constituent parts are acted upon 

 by water or carbonic acid ; and the oxide of iron, 

 which is almost always in its least oxidised state, 

 tends to combine with more oxygen. The con- 

 sequence is, that the feldspar decomposes, and 

 likewise the mica, but the first the most rapidly. 

 The feldspar, which is, as it were, the cement of 

 the stone, forms a fine clay ; the mica, partially 

 decomposed, mixes with it as sand ; and the un- 

 decomposed quai-tz appears as gravel, or sand of 

 different degrees of fineness. As soon as the 

 smallest layer of earth is formed on the surface 

 of a rock, the seeds of lichens, mosses, and other 

 imperfect vegetables which are constantly float- 

 ing in the atmosphere, and which have made it 

 their resting-place, begin to vegetate. Their 

 death, decomposition, and decay afford a certain 

 quantity of organisable matter, which mixes 



