RELATION OF BOCKS TO THB aOIL. 9fXi 



At a, the strata are set up vertically, and are quite 

 thin; suppose them to (litl'er considerably in composi> 

 lion, there would be a ditferent soil in every mile or 

 less. I once examined a series of seven slate rocks, 

 taken from as many ditferent layers of slate, in the 

 same district Four of them were almost destitute 

 of lime, two had about 2 per cent each, and one had 

 nearly 8 per cent How different must have been tlie 

 soils which these slates formed! 



As we descend upon the plain, in the diagram, the 

 strata lie nearly horizontal, and each may perhaps 

 cover a large district. Thas beginning at b, we per- 

 haps come upon a poor sandy soil, formed from some 

 inferior sandstone; proceeding along this for fifty 

 miles, we come at <i,upon a limestone of good quali- 

 ty; here the character of the soil changes at once, 

 and we have a rich, fertile district 



At the points where two diHferent strata meet, is 

 very likely to be a good soil; because a uaiOD (tf the 

 two generally supplies either all that is necaasary to 

 the chemical composition, or altera for the better the 

 physical character of the soil. 



Suppose e, in the hollow, to be an exceedingly wet 

 and tough clay, too tenacious for profitable cmtiva- 

 tion: at the point 6, where we meet the poor aantiy 

 soil before menli»n«i, the sand mixes with the clay, 

 and forms a mellow rich soil. At r, on the hill side, 

 whsre the strata lie horizontally, changes are of course 

 ■Bore frequent, and the character of the soils at Ham 

 base is apt to be adected by washing down from thos» 

 above. 



These differences in the character of different strata, 

 explain also some facts relative to the wetness of soils. 

 "We often see the side of a steep hill rety wet If thtt 

 stratum of nnk and soil at c Ik' stifT, and imperrious 

 to water, all the rain which fulls on the country back 

 and higher up will sink till it comes to this stratmi. 



