GEOLOGY. 



of paper,' he says, ' of several different colours, 

 be pasted one on another, then binding them 

 up together into a ridge in the middle, con- 

 ceive them to be reduced again to a level sur- 

 face by a plane so passing through them LS to 

 cutoff all the part that had been raised; let 

 the middle now be again raised a little, and 

 this would be a good general representation of 

 most, if not all, large tracts of mountainous 

 countries, together with the parts adjacent 

 throughout the whole world. From this for- 

 mation of the earth, it will follow that we 

 ought to meet with the same kinds of earths, 

 stones, and minerals, appearing on the surface 

 in long narrow slips, and lying parallel to the 

 greatest rise of any large ridge of mountains, 

 and so, in fact, we find them.' " (Phil. Trans. 

 1760.) 



And this system of layers or strata not only 

 marks the arrangement of the great masses of 

 which our earth is composed, but it is that of 

 the very rocks themselves. It is to us attended 

 with many advantages, such as the formation 

 of springs, the constitution of soils, which 

 last is that alluded to by Dr. Paris, On the Sails 

 of Cornwall. " The phenomenon of stratifica- 

 tion, which is so well characterized in clay 

 slate, I have often regarded as a wise provision 

 of nature to facilitate its decomposition, and to 

 admit the descent of the roots of trees : and 

 this idea is further strengthened when we dis- 

 cover that this structure is almost entirely con- 

 fined to secondary rocks, whose situation and 

 nature render them capable of cultivation : 

 they are all, for instance, resolved into gently 

 undulating hills, and by farther decomposition 

 they form rich and fertile soils. Primitive 

 formations, on the contrary, which possess no 

 such structure, disintegrate into rugged piles, 

 whose declivities are too steep to admit the ac- 

 cumulations of soil, and cannot, therefore, ever 

 constitute the habitable parts of the globe; 

 and, as far as our geological knowledge will 

 allow us to generalize, it would appear that 

 primitive rocks are accumulated towards the 

 poles, whereas the great mass of secondary 

 formations is found to occupy the middle and 

 southern latitudes, principally between the 20th 

 and 55th degrees, which constitute a portion of 

 the globe eminently calculated for the abode of 

 man, and the animals which are subservient 

 to his wants and comforts." 



The farmer must not imagine, as he pursues 

 ' his researches in this very interesting science, 

 that he will find a great variety of earthy sub- 

 stances in the different, often-varying strata of 

 the earth ; "for," as Mr. Brande says very 

 truly, " siliceous, calcareous, and argillaceous 

 substances, either pure, or nearly so, or in a 

 state of mixture, or loosely and indefinitely 

 blended, rather than in strict chemical combi- 

 nation, constitute a very large relative propor- 

 tion of those rocky masses, or scattered or 

 comminuted substances, which form, or have 

 formed, the most exterior constituents of our 

 planet, and of these, considered in the abstract, 

 the chemical aAd mineralogical history is soon 

 told." Of that brief history, however, it will 

 be well for the farmer to have a general know- 

 ledge. Davy, who saw very clearly the im- 

 portance of the science to practical agricul- 

 532 



GEOLOGY. 



! ture, described them with a view to assist the 

 farmer in his tillage operations. 



The formation of a soil from the different 

 strata by natural causes is also well described 

 by the same great chemist. "It is easy," he 

 says, "to form an idea of the manner in which 

 rocks are converted into soils by referring to 

 the instance of soft granite or porcelain granite. 

 This substance consists of three ingredients, 

 quartz, feldspar, and mica. The quartz is 

 almost pure siliceous earth, in a crystalline 

 form. The feldspar and mica are compound 

 substances: both contain silica, alumina, and 

 oxide of iron ; in the feldspar there is usually 

 lime and potassa; in the mica, lime and mag- 

 nesia. 



"When a granitic rock of this kind has been 

 long exposed to the influence of air and water, 

 the lime and the potassa contained in its con- 

 stituent parts are acted upon by water or car- 

 bonic acid ; and the oxide of iron, which is 

 almost always in its least oxidized state, tends 

 to combine with more oxygen; the conse- 

 quence is, the feldspar decomposes, and like- 

 wise the mica, but the first the more rapidly. 

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

 of the stone, forms a fine clay. The mica 

 partially decomposes, mixes with it as sand, 

 and the undecomposed quartz appear 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 seed of 

 lichens, mosses, and other imperfect vege- 

 tables which are constantly floating in the 

 atmosphere, and which have made it their 

 resting-place, begin to vegetate. Their death, 

 decomposition, and decay, afford a certain 

 quantity of organizable matter, which mixes 

 with the earthy materials of the rock; in this 

 improved soil more perfect plants are capable 

 of subsisting ; these, in their turn, absorb 

 nourishment from water and the atmosphere, 

 and, after perishing, afford new materials to 

 those already provided. The decomposition 

 of the rock still continues, and, at length, by 

 such slow and gradual processes, a soil is 

 formed in which even forest trees can fix their 

 roots, and which is fitted to reward the labour 

 of the cultivator." (Ibid. p. 189.) 



That the geological formation of the soil 

 influences to a considerable degree its rela- 

 tions to a fertilizing supply of moisture, was 

 thus noticed by Dr. ^aris when addressing a 

 Cornish Agricultural Society: "There is a 

 popular adage well known to all the members 

 of this society, that 'the land of Cornwall will 

 bear a shower every week-day, and two upon 

 a Sunday,' the fact is, that the shallowness 

 of the soil, and the nature of its rocky substra- 

 tum, render a constant supply of moisture 

 indispensable ; and here we cannot avoid ad- 

 miring the beautiful contrivance of nature in 

 connecting the wants and necessities of the 

 different parts of the creation with the power 

 and means of supplying them ; thus, in rocky 

 countries like Cornwall, where the soil is ne- 

 cessarily greedy of moisture, the very cause 

 which creates this want is of itself capable' 

 of supplying it; for the rocks elevated above 

 the surface solicit a tribute from every passing 

 shower, while in alluvial and champaign 



