ADAPTATION TO AGRICULTURE- 61 



With regard to their adaptation to human uses, it may be 

 stated generally, that the greater number of the most popu- 

 lous and highly civilized assemblages of mankind inhabit 

 those portions of the earth which are composed of secondary 

 and tertiary formations. Viewed, therefore, in their rela- 

 tions to that agricultural stage of human society in which 

 man becomes established in a settled habitation, and applies 

 his industry to till the earth, we find in these formations 

 which have been accumulated, in apparently accidental 

 succession, an arrangement highly advantageous to the cul- 

 tivation of their surface. The movements of the waters, by 

 which the materials of strata have been transported to their 

 present place, have caused them to be intermixed in such 

 manner, and in such proportions, as are in various degrees 

 favourable to the growth of the different vegetable produc- 

 tions, which man requires for himself and the domestic ani- 

 mals he has collected around him. 



The process is obvious whereby even sohd rocks are con- 

 verted into soil fit for the maintenance of vegetation, by 

 simple exposure to atmospheric agency ; the disintegration 

 produced by the vicissitudes of heat and cold, moisture and 

 dryness, reduces the surface of almost all strata to a com- 

 minuted state of soil, or mould, the fertility of which is 

 usually in proportion to the compound nature of its ingre- 

 dients. 



The three principal materials of all strata are the earths 

 of flint, clay, and lime ; each of these, taken singly and in a 

 state of purity, is comparatively barren ; the admixture of a 

 small proportion of clay gives tenacity and fertility to sand, 



often indicate the sources from which these rounded fragments were sup- 

 plied. 



The transport of these mateiials from the site of older formations to 

 llieir place in the secondary series, and their disposition in strata widely 

 extended over the bottom of the early seas, seem to have resulted from 

 threes producing the destruction of more ancient lands, on a scale of mag-^ 

 nitude unexampled among the actual phenomena of moving waters. 



VOL. I. — 6 



