GEOLOGY OF SOIL. 11 



agricultural chemistry — that rocks do not affect 



THE VEGETATION WHICH COVERS THEM. 



21. This is opposed to the geological doctrine 

 of the times, and may seem to be opposed to the 

 statement, section 18. The difference there stated, 

 may be thought to produce corresponding effects in 

 vegetation. This would be true if rocks exerted 

 any influence on soils, due to their chemical consti- 

 tution. A survey of the geographical distribution of 

 plants, used for food, will show that the common 

 doctrine of the chemical influence of rocks on vege- 

 tation, is not so well supported, as to be considered 

 an established principle. 



22. The plants used for food are cultivated on 

 every variety of rock foundation which the earth 

 presents. Then cultivation is limited neither by 

 granitic nor trappean, by fossiliferous nor non -fossil if- 

 erous rocks. Then product varies not more on dif- 

 ferent, than on the same geological formation. Ev- 

 erywhere, over every variety of rock, the cultiva- 

 tion of the food-bearing plants, repays the labor of 

 the farmer. 



23. Surveying Massachusetts, it is evident the 

 grain crops are not influenced by the peculiar rock 

 formations over which they are grown ; for in this 

 State, with the exception of modern volcanic rocks, 



