466 ON THE CONNECTION OF GEOLOGY 



tains consisting of sandstone, has a much less degree of 

 sterility than in those plains, because the surface of the 

 subjacent rock impedes the progress of the water, and 

 consequently retains it in the soil *. It has been suffi- 

 ciently proved by experiments, that plants can grow in 

 pure sand, when furnished with the necessary quantity of 

 water. A subjacent rocky surface has an entirely different 

 effect upon soil which is very retentive of moisture, upon 

 a clay soil for example, as, in that case, the humidity is 



* The reason here assigned is confirmed by some observations de- 

 livered by one of the latest and most intelligent of the English wri- 

 ters on agriculture. " If,*' says Mr Marshall, " the several strata" 

 (viz. the subsoil and base) " are of so loose a texture, as to permit the 

 waters of rains to pass quickly downward, without being in any suf- 

 ficient degree arrested by the soil, the land may be said to be worth- 

 less to agriculture." He adds, " Before we suggest any improve- 

 ment of lands of the latter description, it will be proper to premise, 

 that many of the light sandy soils of Norfolk, which would other- 

 wise be uniformly absorbed to a great depth, have a thin earthy 

 substance, or " Pan," which intervenes between the soil and the 

 subsoil, and which is of such a texture, as to check the descent of 

 rain waters, and thereby retain them the longer in the soil, as well 

 as to prevent the manure it contains from being carried away by 

 their rapid descent ; yet sufficiently pervious to prevent a surcharge 

 of moisture from injuring the produce. To this fortunate circum- 

 stance is principally owing the fertility of the lands of East Norfolk : 

 for wherever this filter happens to be broken by the plough, or 

 otherwise, the soil becomes unfertile, and continues to be so for a 

 length of years." (See Norfolk, vol. i. page 11.) " This fact aptly 

 suggests the expedient of improving, or fresh forming, a filter of 

 this kind ; seeing how capable it is of producing so many valuable 

 advantages ; the more especially, as it is probably the Norfolk pan 

 owes its origin to fortuitous art, rather than to nature." (See Nor- 

 folk, vol. i. page 12.) " A millstone, or other heavy wheel-shaped 

 stone, made to run upon its edge, i the bottom of the plough-furrow 



