WITH AGRICULTURE AND PLANTING. 465 



is, in a great measure, to be attributed to the circum- 

 stances above mentioned. Columella observes, that silex 

 having a moderate covering of earth, preserves to the 

 latter its humidity ; and Palladius repeats the remark. 

 In districts which consist of quartzose rocks, not less 

 than of granitic ones, the surface is often covered with 

 marshes. Porphyritic rocks, on the contrary, which 

 have a remarkable segregation of parts, as well as co- 

 lumnar basalt, let off the water to lower places. Springs 

 are very frequently found at the bottom of basaltic moun- 

 tains ; for the atmospheric waters penetrate by the per- 

 pendicular fissures to the strata on which the basalt rests, 

 and appear at the place where the two rocks meet. 



The effect of different rocks upon the preservation and 

 diminution of the moisture of fertile soil, influences ve- 

 getation in various degrees. The retentive power of the 

 surface of rocks is of the greatest importance, where the 

 soil consists chiefly of sand, through which the water 

 percolates, and passes off' entirely, unless it meets with a 

 stratum of such a nature, as to obstruct its passage, or 

 comes upon a surface of solid rock. The cause of the 

 sterility of sandy plains is not merely their sandy nature, 

 but also the great depth of the mass or rock capable of re- 

 taining the water. The same sand, when covering moun- 



France, as stated by Mr Arthur Young, and in sundry districts of Eng- 

 land, chalk and limestone bottoms are occasionally observed to be re- 

 tentive and wet. Undergrounds, formed of chalk or limestone, have 

 frequently a thin covering of vegetable mould, from their being, in some 

 cases, over close and wet, and in others over open and dry ; the former 

 condition being unfriendly to vegetation and the formation of mould, 

 and the latter too readily permitting its departure when formed, or other- 

 wise favouring the decomposition and waste of that material. 



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