WITH AGRICULTURE AND PLANTING. 479 



and therefore stiff and cold. The variegated sand- 

 stone, with a marly cement, not unfrequently affords a 

 pretty fertile soil ; the quadersandstem, on the contrary, 

 commonly presents a sandy and arid soil. 



Lastly, in the eighth class we shall place those rocks, 

 whether simple or intimately compounded, whose nature 

 is so loose, or whose parts are so separated, that they 

 fall with great facility into an earthy mass, and are also 

 in part mechanically reduced by water. To this class 

 belong the different varieties of marl, slate-clay, basaltic 

 and volcanic tujfa. These rocks, many of which are ex- 

 tensively diffused, are of much importance in the forma- 

 tion of productive soil, although the quality of the earth 

 produced by them varies much, according to their diffe- 

 rent natures. Slate-clay affords an argillaceous soil ; in 

 earth produced by the decomposition of marl, the clay is 

 diminished in proportion to the greater abundance of the 

 calcareous or sandy parts ; while a mixed and very fer- 

 tile soil is usually generated from basaltic and volcanic 

 tufas. 



The various relations which exist in the stratification 

 and position of rocks, have much influence in producing 

 a diversity in the soil formed immediately from their de- 

 composition. This diversity cannot be so great when 

 different rocks of various ages occur in a determinate or- 

 der in horizontal strata ; in which case, the uppermost 

 bed may exhibit a great extent of surface of the same 

 nature. When, on the other hand, strata of rocks of diffe- 

 rent natures, forms, and dimensions, placed at different 

 angles of inclination, and in different directions, appear 

 at the surface, it will easily be understood how it may 



