AGRICULTURAL GEOLOGY. 13 



alumina (which is essential to the consistency of the soil,) 

 me, potash, soda and iron, some containing riiore and some less 

 f each respectively, the alkalies being the most essential, and 

 jmdering a soil rich, as it is termed, in proportion to their amount. 

 \\ addition to the fact here stated, I may observe that the tendency 

 h decompose is also increased in proportion to the percentage of 

 le alkalies contained in the mineral: a rock of pure quartz is 

 ;ted upon very slowly, while one in which felspar and mica exist 



umbles rapidly. 



In applying the preceding facts, it is easy to see how farms and 

 itates should be selected in a primary district. The depth of 

 :il is an important fact, as is well known, but its derivation is 



I other equally important. For its determination, the outcrop of 

 jcks upon hillsides may be examined, and their nature ascertained; 

 Mether their exposed or weathered surfaces are bleached, and softer 

 lin that of a recent fracture ; or whether they are crumbly, and 

 <;posed to disintegrate. If the rocks are hornblende or pyroxenic 

 jeenstone, or a coarse granite with large masses of felspar, we 

 s|ill expect the soil to contain the alkalies or alkaline earths ; and 

 i )y cultivation they become exhausted, we may expect that by deep 

 c, subsoil ploughing a fresh quantity can be brought to the surface 

 f ■ the use of vegetables, and thus a constant reproduction of them 

 ctained from the decomposition of the coarser particles now inter- 

 Exed wuth the deeper soil. Greenstone and trap, from their more 

 r.dy disposition to undergo change, may be ranked among the 

 hit materials for a foundation soil, and possess all the requi- 

 s:?s desired for the cultivation of grains and fruits. They are not 

 Siporous as the granitic sands that are termed leechy ; nor so com- 

 P-t as many of the argillaceous soils, many of which retain the 

 i^jter in pools upon the surface. 



§ 5, Drifted soil. 

 V farther consideration of the causes which have distributed 

 tl soil and spread the debris of rocks at a distance, is of some 



II )ortance while treating of the northern counties ; as it may ap- 

 p<r to those who are familiar with the drift or diluvial theories, 

 tit little reliance can be placed upon our instructions for 

 derraining the character of the soil by observing the rocks be- 



