ARBORICULTURE. 317 



CHAP. I. 



Of Earths and Soils. 



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 organized 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 ; next the manner 

 in which earths and soils are produced ; and lastly we shall 

 consider in succession the nomenclature and qualities of soils. 



Of the Geological Structure of the Globe. 



The materials of which the general mass of this earth is 

 composed, are variously distributed in different parts. In some 

 places they form irregular masses or blocks, either buried below 

 the surface, or elevated to a greater or less height above it. Tn 

 most places, however, the materials are arranged in a more 

 regular manner ; those of the same kind being collected into 

 extensive masses, lying in layers or strata, above or below a 

 similar mass of another kind, or these alternate with each other 

 to a considerable depth. These strata are sometimes found 

 arranged in a direction parallel to the horizon ; at others they 

 are vertical, or perpendicular to the horizon, appearing as if the 

 horizontal strata had been lifted up, and laid upon their edges. 

 More commonly the strata are arranged in a direction inclining 

 to the horizon, when they are said to dip. 



The uppermost stratum is in most places covered to a certain 

 depth with mould that has evidently been formed from the 

 decomposition of organized substances. In many parts of the 

 earth this mould extends to a very considerable depth, and 



