460 ON THE CONNECTION OF GEOLOGY 



ford a more easy passage to roots, than granular crystal- 

 line ones ; pure quartz resists the roots of plants in the 

 highest degree ; sandstone much less ; and pure limestone, 

 on account of its comparatively small number of fissures, 

 is much less favourable to vegetation than marl, chalk, 

 or slightly cohering calcareous rocks, the masses of which 

 are usually split in all directions. 



The direction and inclination of the strata have also 

 some influence in this matter ; for, in proportion as the 

 principal fissures of the strata are, from their direction 

 or inclination, more readily presented to the roots of ve- 

 getables, the less prejudicial will their surface be to ve- 

 getation. Horizontal strata, therefore, are the least fa- 

 vourable to vegetation, perpendicular ones the most. 

 In the inclination of strata intermediate in some degree 

 between these positions, the roots of vegetables will find 

 a greater obstacle on the side of a hill in which the sur- 

 face of a stratum is opposed to them, than on the other, 

 in which the principal fissures of the strata are open. 

 The effects of this circumstance may frequently be ob- 

 served in mountainous tracts having two principal incli- 

 nations, the state of vegetation, and especially the growth 

 of wood, being more prosperous on the one of these de- 

 clivities than on the other. 



The surface of the solid strata of the earth may also 

 have an indirect influence upon the cultivation of vege- 

 tables. The various inclinations of this surface deserve 

 first to be considered, being of the greatest effect with 

 regard to fixing the fertile soil. The horizontal position 

 of a rocky surface is in the highest degree favourable to 

 the stability of vegetable earth ; and the greater its an- 

 gle of inclination, the greater is the danger of its losing 



