20 PHYSICAL GEOGRAPHY AND PREVAILING WINDS. 



of rich marl, which may be usefully employed in subduing the 

 stiff er clays. 



In many other parts of this and of nearly all other countries, 

 drifted materials cover the true living rocks of a district, and 

 modify, if they do not actually form, the entire cultivated soil, 

 as well as the subsoil which lies below it. Not only, therefore, 

 must the local geology, properly so called, of any locality in 

 which experiments are to be made, or when made to be inter- 

 preted, be an element in the knowledge necessary to the sug- 

 gester, or to the careful weigher and criticiser of experiments, 

 but an acquaintance with the nature of the local drift must be 

 regarded as especially desirable. 



3. Nor are the physical geography and prevailing winds of 

 much less importance. The rains bring down from the high 

 grounds, either in suspension or in solution, substances which 

 enrich the bottoms and valleys at the expense of the hills and 

 slopes. The sea-wind, again, drives inland the salt spray which 

 it lifts from the tops of the curling waves, and sprinkles the sea- 

 rain over the surface, with all it holds in solution. The presence 

 of hills arrests it, and shelters the inland slopes and valleys 

 from the genial showers ; and thus a difference, depending upon 

 physical structure alone, is established between the soils on the 

 opposite sides of the sheltering hills. 



A coast is girt with accumulations of broken shells. Where 

 the land is flat, as in some of our western isles, the wind sweeps 

 the light fragments far inland, and makes the herbage as sweet 

 as on our limestone hills, and the soil as rich in lime. But, 

 where the land rises suddenly from the shore, the shell drift is 

 arrested, and the soil retains the composition due to its geolo- 

 gical origin, its natural moisture or dryness, and its nearness to 

 the sea. 



Thus physical geography and prevailing winds become im- 

 portant considerations to the experimental agriculturist. 



9. The composition of local streams and spring s^ and of 

 local rocks. 



The soil of every field or district is watered by streams or 

 springs of a local character, which flow over or percolate certain 



