TO THE ROCKS ON WHICH THEY REPOSE. 219 



other known facts. It appears that rocks must yield to atmospheric influence, and the 

 more so as their surface is increased ; and hence upon rough surfaces the effects are far 

 greater than on smooth ones, and still greater where the natural joints are open and admit 

 water, which, on freezing, exerts its ordinary effects by expansion ; and as these effects 

 continue, the most stable materials are finally broken up and removed ; and when com- 

 pletely reduced to soil, they have already lost a large part of their soluble matter, whatever 

 it may have been. The debris is then composed of the most insoluble parts or elements, 

 as silica and the silicates, alumina, and oxide of iron; and the probability is that all soils 

 ■would, in the end, other things being equal, be reduced to about the same state. If two 

 kinds of soil were treated with water, or washed upon a filter, the soluble matter would 

 soon be removed from each, and they w T ould be reduced to about the same value. The 

 difference in the value of soils is often preserved by the natural vegetation, an effect due 

 to the power, which vegetables possess, of taking up by their roots the soluble matter, and 

 conveying it to the surface ; and so long as a soil is covered with a natural vegetation, no 

 matter how heavy or how rank it is, the surface grows richer. By this means a certain 

 amount of inorganic matter, essential to vegetation, will be always preserved at the surface, 

 provided it is not ploughed or put under artificial cultivation ; for then, aside from what 

 is removed, the ploughing and stirring of the soil exposes it to the water, which percolates 

 through it, carrying the soluble matter from the surface beyond the reach of roots. The 

 result then is, that a soil differs more and more from the rock from which it is derived, by 

 gradually losing some of the elements which were contained in the rock. What the rock 

 does not contain, will be absent from the soil, but the proportions will vary. Knowing 

 then the composition of the rock, we only know what the soil probably contains, and what 

 it certainly does not ; making due allowance for the loss of soluble matter, which it must 

 sustain under a course of cultivation. 



The amount of material essential to the growth of good crops, can be learned only from 

 analysis. The information to be derived from the rock beneath, embraces that knowledge 

 which concerns the kind of elements, and not their amount, except in those cases where 

 there is always a supply. Silex, and probably alumina and iron, are so generally diffused, 

 that it is not difficult to determine the fact of their presence or absence by mere inspection. 

 One important effect which has not been fully stated in regard to the transportation of 

 the soils of New-York, is this : the softer rocks have been made to contribute largely to 

 those of the harder ones. The harder rocks, in the first place, resisted the force of the 

 diluvial current ; they checked its force, and hence the debris which was borne along was 

 deposited at those places where the resistance was the greatest. It is for this reason that 

 the north and northwest slopes are coated with an enormous depth of soil. The slopes of 

 Livingston county have a greater amount of soil from the Onondaga shales, or Salt group, 

 than Onondaga county itself. The wheat clays and wheat sands of Livingston came 

 mostly from the Salt group, and the soil is deeper and more abundant than in Onondaga. 



28* 



