RELATION OF ROCKS TO THE SOIL. 203 



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

 thin; suppose them to differ considerably in composi- 

 tion, there would be a different soil in every mile or 

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

 taken from as many different 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 the 

 soils which these slates formed! 



As we descend upon the plain, in the diagram, t> . 

 strata lie nearly horizontal, and each may perha * •„ 

 cover a large district. Thus beginning at 6, we per- 

 haps come upon a poor sandy soil, formed from some 

 inferior sandstone; proceeding along this for fifty 

 miles, we come at d, 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 different strata meet, is 

 very likely to be a good soil; because a union of the 

 two generally supplies either all that is necessary to 

 the chemical composition, or alters 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 cultiva- 

 tion: at the point b, where we meet tne poor sandy 

 soil before mentioned, the sand mixes with the clay, 

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

 where the strata lie horizontally, changes are of course 

 more frequent, and the character of the soils at the 

 base is apt to b- 1 affected by washing down from those 

 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 very wet. If the 

 stratum of rock and soil at c be stiff, and impervious 

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

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



