WITH AGRICULTURE AND PLANTING. 467 



increased to a prejudicial degree. In land of this nature, 

 a substratum of rock having the property of drawing off 

 the water would be useful. 



The different conditions of rocks with regard to calo- 

 ric, may have some indirect influence upon the vigour of 

 plants. Heat, whether imparted to the vegetable soil by 

 the sun's rays, or generated by various chemical processes 

 in the earth itself, penetrates to the surface of the sub- 

 jacent rocks, and is more or less drawn from it in a long- 

 er or shorter time. Columella observes, that rocks in 

 the upper part of the soil are prejudicial to vines and 

 trees, but in the lower part cool them. The heat of 

 soil will be more or less drawn from it, according to 

 the greater or less conducting power of the subjacent 

 rock. Compact crystalline rocks are probably better 

 conductors of caloric than those which are of looser tex- 

 ture ; siliceous rocks than argillaceous and calcareous 

 ones. The influence of the subjacent rock must be 

 greater in this respect, in proportion to the thinness of 

 the superincumbent soil. The effect of the abduction 

 of caloric is more particularly sensible, where the roots 

 of cultivated plants touch the rock, a circumstance 



(the thickness of its edge being equal to the width of the furrow), 

 by the help of an axle and wheels, would greatly compress a light, 

 porous subsoil. The idea of forming a pan artificially, struck me 

 first in Norfolk ; and time and experience have strengthened it. If 

 the experiment be made on a compressible subsoil, as sandy loam, 

 or the soft rubble which sometimes intervenes between an absorbent 

 soil and an open rock, there can be little doubt of its success. But 

 on loose open gravel, which is not sufficiently mixed with tenacious 

 mould to sheath it, and lying on an open base, less utility may be ex- 

 pected from it." 



G2 



