Book III. CLASSIFICATION OF SOILS. 313 



modifying circumstance of pressure. All the members of these formations are not 

 every where to be found : sometimes one or more species of rock may be wanting in 

 the series; but a skilful geologist can generally detect a wonderful degree of regularity 

 in the superposition of strata, which, to an unpractised eye, present only a mass of 

 confusion. 



2107. The relative situation of these rocks in Britain is as follows : The primitive rocks 

 are usually observed constituting a portion of the most elevated parts of the surface of the 

 earth ; the rocks of transition usually form the less elevated ridges ; the flo'e'tz rocks, with 

 alluvial matter, generally constitute the bases of plains, or of an undulated country. The 

 two latter formations constitute by far the greatest portion of England and the low parts 

 of Scotland : the mountains of Cumberland and Wales are chiefly composed of rocks 

 of transition, while Cornwall and the Highlands of Scotland have generally a basis of 

 primitive rocks, over which some rocks of the transition series are occasionally super- 



imposcd. 



2108. The original authorities for the geological distribution of English strata are 

 Smith's Map and Sections; Greenough's Map; Coneybeare's and Phillips s Geology of 

 England ; SedgewicJee's papers in the Gcologiccd Transactions ; Webster's Isle of Wight, &c. 

 These are all authorities of weight with mineralogists. 



2109. The surface earth, or that which forms the outer coating of the dry parts of the 

 globe, is formed by the detritus, or worn off parts of rocks and rocky substances. For 

 in some places, as in chasms and vacuities between rocky layers or masses, earth occupies 

 many feet in depth ; and in others, as on the summits of chalk hills or granite mountains, 

 it hardly covers the surface. 



2110. Earths are therefore variously composed, according to the rocks or strata which have supplied their 

 particles. Sometimes they are chiefly formed from slate-rocks, as in blue clays ; at other times from 

 sandstone, as in silicious soils ; and mostly of a mixture of clayey, slaty, and limestone rocks, blended in 

 proportions as various as their situations. Such we may suppose to have been the state of the surface of 

 the dry part of the globe immediately after the last disruption of its crust ; but in process of time the decay 

 of vegetables and animals forms additions to the outer surface of the earths, and constitute what are called 

 soils ; the difference between which and earths is, that the former always contain a portion of vegetable 

 or animal matter. 



2111 The manner in which rocks are converted into soils, Sir H. Davy observes (Elem. of Jgrtc. Client., 

 188.), may be easily conceived 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 silicious 

 earth in a crystalline form. The feldspar and mica are very compounded substances ; both contain silica, 

 alumina, and oxide of iron ; in the feldspar there is usually lime and potassa ; 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 the potassa 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 

 consequence 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 tine clay : the mica, partially decom- 

 posed, mixes with it as sand ; and the undecomposed quartz 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 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 organisable 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 labours of the cultivator. 



2112. The formation of peaty soils is produced from very opposite causes, and it is interesting to 

 contemplate how the same effect may be produced by different means, and the earth which supplies almost 

 all our wants may become barren alike from the excessive application of art, or the utter neglect of it. 

 Continual pulverisation, and cropping, without manuring, will certainly produce a hungry barren soil ; 

 and the total neglect of fertile tracts will, from their accumulated vegetable products, produce peat soils 

 and bogs. Where successive generations of vegetables have grown upon a soil, Sir H. Davy observes, 

 unless part of their produce has been carried off bv man, or consumed by animals, the vegetable matter 

 increases in such a proportion, that the soil approaches to a peat in its nature : and if in a situation where 

 it can receive water from a higher district, it becomes spongy and permeated with that fluid, and is gene- 

 rally rendered incapable of supporting the nobler classes of vegetables. 



21 lo. Spurious peaty soil. Lakes and pools are sometimes filled up by the accumulation of the remains 

 of aquatic plants; and in this case a sort of spurious peat is formed. The fermentation in these cases, 

 however, seems to be of a different kind. Much more gaseous matter is evolved ; and the neighbourhood 

 of morasses, in which aquatic vegetables decompose, is usually aguish and unhealthy; whilst that of the 

 true peat, or peat formed on soils originally dry, is always salubrious. 



2114. Soils may generally be distinguished from mere masses of earth by their friable 

 texture and dark colour, and by the presence of some vegetable fibre or carbonaceous 

 matter. In uncultivated grounds, soils occupy only a few inches in depth on the sur- 

 face, unless in crevices, where they have been washed in by rains ; and in cultivated soils 

 their depth is generally the same as that to which the implements used in cultivation 

 have penetrated. 



211.5. Much has been written on soils, and, till lately, to very little purpose. All the 

 Roman authors on husbandry treated the subject at length ; and in modern times, in this 

 country, copious philosophical discourses on soils were published by Bacon, Evelyn, 

 Bradley, and others; but it may be truly said, that in no department of cultivation was 

 ever so much written of which so little use could be made by practical men. 



