312 SCIENCE OF AGRICULTURE. Part IT. 



Chap. I. 

 Of ^Earths and Soils. 



2100. 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 decomposed 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. We shall next consider in succession 

 the Nomenclature, Quality, Use, and Improvement of Soils. 



Sect. I, Of the Geological Structure of the Globe and the Formation of Earths and Soils. 



2101. The crust of our earth, when examined, will be found to be composed of 

 various stony bodies, differing in their structure and composition. Some of these are 

 arranged in strata of greater or less regularity, and more or less inclined to the horizon ; 

 others show no marks of stratification, but constitute large mountain masses, without 

 any definite shape, or fill up fissures in other rocks, forming veins. Some rocks show an 

 evident compound or aggregated structure ; others appear, to the naked eye, of a uniform 

 texture : some stony bodies contain undoubted remains of animals and vegetables, which 

 chiefly belong to species of organised beings no longer known to exist in a living state ; 

 other rocks are always destitute of every trace of organised remains. These peculiarities 

 have given rise to different classifications of rocks. One sect of geologists divide rocks 

 into simple and compound ; and again subdivide these classes according as the structure 

 of the rock is compact, granular, slaty, porphyritic, or amygdaloidal. The greatest number 

 of geologists, however, are not satisfied with that arrangement, but have ventured to 

 speculate on the relative age or era of the formation of the different kinds of rock. The 

 data on which they proceed are, chiefly, the presence or absence of organic remains, and 

 the superposition of one kind of rocky bed on another. All geologists are agreed in con- 

 sidering stratified rocks as arranged and deposited by the agency of water, and therefore 

 the relative age of such rocks may be generally inferred from their relative position ; but 

 philosophers differ both with regard to the origin and era of the unstratified rocks, and 

 also of the minerals which occupy veins. It is not our business here to enter into this 

 discussion, but we shall content ourselves by a slight sketch of the most generally received 

 arrangement of rocks, which, though it involves theoretic considerations, is convenient to 

 the student of mineralogy. The crust of our globe may be considered as composed of 

 five series of rocks : primitive, transition, floetz, alluvial, and volcanic. 



2102. Primitive rocks. These, from the absence of organic remains, are conceived to 

 have been deposited, in their present situation, before the creation of animals, and, from 

 most usually lying below other rocks, are supposed to be the most ancient. Of these the 

 chief species are granite (including syenite), gneiss, mica slate (including talc slate), clay 

 slate, primitive limestone, primitive trap, serpentine, quqrtz rock, and some kinds of 

 porphyry. 



2103. Rocks of transition. In these a few organic remains occur, but neither fre- 

 quently nor in large quantity. They are supposed to have obtained their present form 

 during the transition of the surface of the earth from a chaotic to a habitable state. The 

 principal members of this series are greywacke, one kind of limestone, and occasionally 

 most of the rocks of the first series. 



2104. Floetz rocks are so named from their generally occurring in nearly horizontal 

 strata. They were formerly termed secondary, in contradistinction to the primitive 

 series, and they constitute the terrain secondaire of the French geologists. The principal 

 rocks of this class are sandstone or freestone, which appears to be of different ages, though 

 comprehended still in the floetz series ; limestone (including alpine limestone, inagnesian 

 limestone, oolite, chalk, gypsum, and the calcareous beds of the Paris basin), coal, and the 

 accompanying rocks of our great coal-fields ; trap rocks, including basalt, wacke, and the 

 great body of kindred rocks, which often form the summits of considerable hills. 



2105. Alluvial deposits, chiefly consisting of beds of clay, sand, gravel, and some 

 cemented rocks. The first three formations appear to be universally distributed over the 

 globe, and are supposed to owe their formation to causes acting before the land had yet 

 appeared above the waves. The alluvial formations are conceived to be produced by the 

 action of water on the rocks already mentioned. 



2106. Volcanic rocks. Of this series different kinds of lava, scoria, puzzuolana, &c., 

 are undoubted members ; and most geologists now include in it certain varieties of trap, 

 trachyte, obsidian, and pumice ; while others are disposed to consider all trap rocks, and 

 even granite, as the products of either recent or ancient volcanic fire, acting under the 



