Book II. NATURAL AGENTS OF VEGETABLE CULTURE. 



217 



BOOK IL 



OF THE NATURAL AGENTS OF VEGETABLE GROWTH AND CULTURE. 



1017. The phenomena of vegetation being examined, and the fact ascertained that plants 

 derive their nourishment from the external elements of matter : the next step in the study 

 of tlie science of gardening is to enquire into the composition and nature of material bodies^ 

 and the laws of their changes. The earthy matters which compose the surface of the earth, 

 the air and light of the atmosphere, the water precipitated from it, the heat or cold pro- 

 duced by the alternation of day and night, and by chemical composition and resolution, 

 must include all the elements concerned in vegetation. These elements have all been 

 necessarily brought into notice in the study of the vegetable kingdom ; but we shall now 

 examine more minutely their properties, in so far as they are connected with cultivation. 

 To study them completely, reference must be had to systems of chemistry and mechanical 

 philosophy, of which those of Dr. Thomson (St/stem of Chemistry,) and Dr. Young 

 {Lectures on Mechanical Fhilosophi/,) may be especially recommended. 



Chap. I. 



Of Earths and Soils. 



1018. Earths are the productions of the rocks which are exposed on the surface of the 

 globe, and soils are earths mixed with more or less of the decomjwsed organised matter 

 afforded by dead plants and animals. Earths and soils, therefore, must be as various as 

 the rocks which produce them, and hence to understand their nature and formation it is 

 necessary to begin by considering the geological structure of the territorial surface, and 

 the manner in which earths and soils are produced ; and we shall next consider in suc- 

 cession the nomenclature, quality, use, and improvement of soils. 



Sect. I. Of the Geological Structure of the Globe and the Formation of Earths and SoUs» 



1019. The crust, or under surf ace of t'he earth, is considered by geologists as presenting 

 four distinct series of rocky substances ; the first, supposed to be coeval with the world, 

 are called primitive, and consist cliiefly of granite and marble, below which man has not 

 yet penetrated. The second series, called by the Wernerians transition-rocks, are of more 

 recent formation, and seem to have resulted from some great catastrophe, (probably that 

 to which history gives the name of deluge,) tearing up and modifying the former order 

 of things. Clay-slate is one of the principal rocks of this class, and next limestone, 

 sandstone, and trap or whinstone. The third series are called secondary rocks, and 

 seem to owe their formation to partial or local revolutions, as indicated by their compa- 

 ratively soft and fragile structure, superincumbent situation, and nearly horizontal position. 

 They are chiefly limestones, sandstones, and conglomerations of fragments of other rocks, 

 as plum-pudding-stone, &c. and appear rather as mechanical deposits from water than 

 as chemical compounds from fusion or solution. A fourth stratum consists of alluvial or 

 earthy depositions from water, in the form chiefly of immense beds of clays, marls, or 

 sands. These strata are far from being regular in any one circumstance ; sometimes one 

 or more of the strata are wanting, at other times the order of their disposition seems par- 

 tially inverted ; their continuity of surface is continually interrupted, so that a section of 

 the earth almost every where exhibits only confusion and disorder to persons who have not 

 made geology more or less their study. 



1020. The succession of alluvial, secondary, transition, and primary strata, in England, 

 has been illustrated by Professor Brande {Outlines of Geology), by two sections, supposed 

 to be taken through them. 



1021. The first section {fig. 72.) commences with the blue clay of London (1), and pro- 

 ceeding westward through the counties of Berkshire, Hampshire, Wiltshire, Dorsetshire, 

 and Devonshire, terminates at the Land's End, in Cornwall. The rocks and earths pre- 

 sented in this line are, the Windsor alluvion (2), Hampshire and Salisbury chalk (3), 

 alluvion (4), sandstone (5), alluvion (6), Sherborne freestone (7), sandstone (8), blue ' 

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

 limestone (12), Dartmoor slate (13), granite (14), slate again (15), greenstone (16), 



( ornwall serpentine (17), slate killas (18), Cornwall granite (19), slate killas (20), and 

 finally, Cornwall granite. 



72 



