ARBORICULTURE. 319 



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, sand stone, and 

 trap or whin stone. The third series are called secondary rocks, 

 and seem to owe their formation to partial or local revolutions, 

 as indicated by their comparatively soft and fragile structure, 

 superincumbent situation, and nearly horizontal position. They 

 are chiefly lime stones, sand stones, and conglomerations of 

 fragments of other rocks, as plum-pudding stone, etc., 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 partially 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. 



Of the Formation of Earths and Soils. 



The surface earth, or that which forms the outer coating of 

 the dry parts of the globe, is formed by the detrious 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 summit oi 

 chalk hills or granite mountains, it hardly covers the surface. 

 Earths are, therefore, variously composed, according to the rocks 

 or strata which have supplied their particles. Sometimes they 

 are chiefly formed from slate rocks, as in blue clays ; at other 

 times from sand stone, as in siliceous soils ; and mostly of a 

 mixture of claey, slaty and lime stone 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 



