218 



SCIENCE OF GARDENING. 



Part II. 



1022. The second section {jig. 73.) commences with the coal strata, and limestone 

 resting upon slate and granite in Cumberland, and thence proceeds towards the metropolis 

 by Yorkshire, Derbysliire, Leicestershire, Northamptonshire, Bedfordshire, and Hert- 

 fordshire. The passage is here exhibited from the primaiy rocks of Cumberland to the 

 secondary hills of the southern counties. It shows the Cumberland coal (a), limestone 

 and slate (b), the Mossdale granite (c), slate (rf), grauwacke (e), Ribblesdale limestone 

 (y)' gritstone (g), Ashton coal (Ji), Derby limestone (i), Derby toadstone (k), gritstone 

 (/), gypsum {in), sandstone {n), limestone (o), Charnwood slate (/>), Mountsorrel 

 granite {q), red sandstone (r), lias limestone (5), Northampton oolite or freestone (i), 

 Woburn sand {u), Dunstable chalk {v), and terminates in the London clay {w), with 

 wliich the first section sets out. 



73 



1023. The surface earth, or that wliich 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. 



1024. Earths are, therefore, variously composed, according to the rocks or strata which 

 have supplied their particles. Sometimes they are chiefly foi-med from slate-rocks, as in 

 blue clays ; at other times from sandstone, as in siliceous 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 pi-ocess of time the decay of ve- 

 getables 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 fonner always contain 

 a portion of vegetable or animal matter. 



1025. The majiner in which rocks are converted into soils, Sir H. Davy observes {Elem. 

 of J.gric. 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. Tlie quartz is almost pure siliceous earth in a crystalline form. The feld- 

 spar and mica are veiy compounded 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 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 witli more oxygen ; the consequence is, that the feldspar decom- 

 poses, and Ukewise the mica; but the first the most rapidly. The feldspar, wliich is as it 

 were the cement of the stone, forms a fine clay : the mica partially decomposed mixes 

 \yith it as sand ; and the undecomposed quartz appears as gravel, or sand of different de- 

 grees 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, dGComposition, and decay afford a ceitain 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 pro\'ided : the n 

 decomposition of the rock still continues ; and at length, by such slow and gradual pro- 

 cesses, a soil is formed in which even forest-trees can fix their roots, and which is fitted to 

 reward the labors of the cultivator. 



1026. The formation of peaty sails is produced from very Opposite causes, and it is interesting to contem- 

 plate how the same effect may be produced by different means, and the earth which suppUes ahnost all 

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

 tinual 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 offby 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 gradually 

 rendered incapable of supporting the nobler classes of vegetables. Many peat-mosses seem to have been 

 formed by the destruction of forests, in consequence of the imprudent use of the hatchet by the early cul- 

 tivators of the country in which they e::ist : when the trees are felled in the outskirts of a wood, those in 

 the interior are exposed to the inQuence of the winds ; having been accustomed to shelter, they become 

 unhealthy, and die in their new situation ; and their leaves and branches gradually decomposing, produce 

 a stratum of vegetable matter. In many of the great bogs in Ireland and Scotland, the larger trees that 



