272 



THE SOILS OFTEN CHANGE FROM SAND TO CLAT. 



presented — in which case it is difficult to decide whether it should be con- 

 sidered as the under or tlie upper clay — though in other spots both sand 

 and clay, or gravel and clay, present themselves. 



It will at once occur to you from the inspection of this diagram, that 

 the general character of the soil in the county of Durham, whenever 

 such accumulations of drifted matter occur, is not to be judged from the 

 nature of the rocks on which they are known to rest. 



Another fact, not unworthy of your attention, is the rapid alternations 

 of light and heavy soil, of sands or gravels and clays, which present 

 themselves in the same district, I may say in the same farm, and often 

 in the same field. This arises from the irregular thickness of the de- 

 posit of sand or gravel over which the upper clay rests. The surface 

 of this sand is undulating, as if it had formed a country of round hills 

 before the clay was deposited upon it. This appears in the following 

 diagram, which represents the way in which the several layers are seen 

 to occur in the Crindon cut on the Hartlepool railway : — 



Here 1 is the magnesian lime-stone, not visible ; 2, the under clay, 

 with boulders ; 3, the sand rising in round hills, and here and there 

 piercing to the surface ; and 4, the upper boulder clay. 



In the county of Durham it is a very usual expression that the tops 

 of the hills are light turnip soil — but that they fall off to clay. Both the 

 meaning and the cause of this are explained by the above diagram. 



Nor is this mode of occurrence rare among the alternate sands and 

 clays of which the superficial accumulations in various parts of the 

 country consist. Nearly the same circumstances give rise to the rapid 

 changes so frequently observed in the character of the soil, as we pass 

 from field to field, not in this county only, but i/i various other parts ot 

 our island. 



§ 7. How far these accumulations of drift interfere ivith the geiieral 

 deductions of Agriculcural Geology. 



Thus it appears, that over the eastern half of the county of Dur- 

 ham, and over large portions of other counties, the soils are found to 

 rest upon and to ilerive their character from accumulations of drifted 

 materials more os lese different in their nature from the rocks that lie 

 beneath. 



But in the precedkig lecture I have endeavoured to show you that 

 soils are derived from the rocks on which they rest, and to impress upon 

 you the close general relation which exists between the kind of rocks of 

 which a country is composed, and the kind of soils by which its surface 

 is overspread. 



How are these apparent contradictions to be reconciled ? How is any 



