AGRICULTURALGEOLOGY. 6 



narbles used in the construction of monuments, and by that of 

 )ther rocks when carefully smoothed ; whereas upon the exposed 

 iurfaces of quarries, the sloping sides are often deeply grooved by 

 he water which slowly trickles over their surfaces. 



Granite and gneiss disintegrate and decompose from their pecu- 

 iar chemical composition, and the presence of alkalies in the fcd- 

 'par and mica exert a powerful influence in these changes. High 

 rranitic peaks in the region of frosts undergo a rapid decay, and 

 n consequence furnish upon the slopes and in the valleys beneath, 

 heir peculiar soils, which are well adapted to grass and grain. 

 The alkalies in these rocks, if completely insulated, would pass 

 iflf rapidly through the soft materials, and be lost to vegetation. 

 They are, how^cver, so combined wnth silica, that they are com- 

 laratively unaffected by the common solvent, water, and hence 

 re retained in the soil for the use of plants. 



Other kinds of rocks liable to decay, are the siliceous limestones, 

 ne of which is the calciferous sandstone. It appears from cxami- 

 ation that the lime is dissolved out, leaving upon the surface the 

 ilex in grains, which falls off by its own weight, or else is rubbed 

 fF by friction. The dissolved lime, however, does not all pass 

 ito and remain in the soil, but is carried down, and forms very 

 •equently with other materials a hardpan^ a puddingstone, or con- 

 retions, the lime acting as a cement j in other instances it perco- 

 ites into and through the rock, and forms stalactites, veins or 

 ther deposits. The same action or power which dissolves the 

 arbonate of lime in solid rocks, dissolves also that w'hich may be 

 liffused through the soil. This takes place where the surface is 

 fequently stirred, as in cultivated fields. Thus this element is re- 

 lieved both by vegetation and by the ordinary action of rain 

 'ater, and hence its deficiency in most of the soils of New- York 

 id New-England. 



^ 2. Classification of rocks. 



The classification of rocks has been a most perplexing study to 

 :ologists. They have not disagreed, however, so much as to 

 e planes where lines of separation should be drawn, as in the 



signation of the masses. The ancient names, primitive^ transi- 

 on and secondary, have ;dl been objected to, and have been aban- 



