310 



SCIENCE OF AGRICULTURE. 



Part II. 



End, in Cornwall. The rocks and earths presented in this line are, the Windsor alluvion (2), Hampshire 

 and Salisbury-chalk (3), alluvion (4), sandstone (5), alluvion (6), Sherborne freestone (7), sandstone (8), 



245 



r? y^ - y /-- 



blue lias limestone (9), Blackdown sandstone (10), Devonshire red sandstone (11), mountain limestone 

 (12), Dartmoor slate (13), granite (14), slate again (15), greenstone (16), Cornwall serpentine (17), slate 

 killas (18), Cornwall granite (19), slate killas (20), and finally Cornwall granite. 

 2058. The second section (^g. 246.) commences with the coal strata, and limestone resting upon slate 



246 



and granite in Cumberland, and thence proceeds towards the metropolis by Yorkshire, Derbyshire, 

 Leicestershire, Northamptonshire, Bedfordshire, and Hertfordshire. The passage is here exhibited from 

 the primary rocks of Cumberland to the secondary hills of the southern counties. It shows the Cumber- 

 land coal (a), limestone and slate (6), the Mossdale granite (c), slate (d), grauwacke (e), Ribblesdale lime- 

 stone (/), gritstone (ff), Ashton coal (h), Derby limestone (i), Derby toadstone (k), gritstone (Z), gypsum 

 {m), sandstone (n), limestone (o), Chamwood slate (p), Mountsorrel granite (g), red sandstone (r), lias 

 limestone (s), Northampton oolite or freestone (t), Woburn sand (m), Dunstable chalk (v), and terminatea 

 in the London clay (t), with which the first section sets out 



2059. 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 otliers, as on the 

 summits of chalk hills or granite mountains, it hardly covers the surface. 



2060. 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 siliceous soils ; and mostly of a mixture of clayey, slatey, 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 form 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 por- 

 tion of vegetable or animal matter. 



2061. The manner in which rocks are converted into soils. Sir H. Davy observes {Eletn. of Agric. Chem. 

 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 siliceous 

 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 fine 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 labors of the cultivator. 



2062. The formation of peaty soils is produced from very opposite causes, and it is interesting to con- 

 template 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 by 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. 



2063. Spurious peaty soil. Lakes and pools of Water 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 neigh- 

 borhood 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. 



2064. Soils may generally be distinguished from mere masses of earth by their friable texture, dark color, 

 and by the presence of some vegetable fibre or carbonaceous matter. In uncultivated grounds, soils 

 occupy only a few inches in depth on the surface, 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. 



2065. 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 modem 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 prac- 

 tical men. 



Sect. II. Classification and Nomenclature of Soils. 



2066. Systematic order and an agreed nomencluture are as necessary in the study of soils as of plants 

 or animals. The number of provincial terms for soils which have found their way into the books on 

 cultivation, is one reason why so little use can be made of their directions. 



2067. ^ correct classification of soils may be founded on the presence or absence of 

 organic and inorganic matter in their basis. This will form two grand classes, viz. 

 primitive soils, or those composed entirely of inorganic matter, and secondary soils, or 

 those composed of organic and inorganic matter in mixtures. These classes may be 



