DECOMPOSITION OF DIFFERENT ROCKS. 35 



Granite and gneiss disintegrate and decompose from their peculiar chemical composition, 

 and the presence of alkalies in the felspar and mica exert a powerful influence in these 

 changes. High granitic peaks in the region of frosts undergo a rapid decay, and in con- 

 sequence furnish upon the slopes and in the valleys heneath their peculiar soils, which 

 are well adapted to grass and grain. The alkalies in these rocks, if completely insulated, 

 would pass off rapidly through the soft materials, and be lost to vegetation. They are, 

 however, so combined with silica, that they are comparatively unaffected by the common 

 solvent, water, and hence are retained in the soil for the use of plants. 



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

 calciferous sandstone. It appears from examination that the lime is dissolved out, leaving 

 upon the surface the silex in grains, which falls off by its own weight, or else is rubbed off 

 by friction. The dissolved lime, however, does not all pass into and remain in the soil, 

 but is carried down, and forms very frequently with other materials a hardpan, a pudding- 

 stone, or concretions, the lime acting as a cement ; in other instances it percolates into and 

 through the rock, and forms stalactites, veins or other deposites. The same action or power 

 which dissolves the carbonate of lime in solid rocks, dissolves also that which may be dif- 

 fused through the soil. This takes place where the surface is frequently stirred, as in cul- 

 tivated fields. Thus this element is removed both by vegetation and by the ordinary action 

 of rain-water, and hence its deficiency in most of the soils of New-York and New-England. 



§ 2. Classification of rocks. 



The classification of rocks has been a most perplexing study to geologists. They have 

 not disagreed, however, so much as to the planes w T here lines of separation should be 

 drawn, as in the designation of the masses. The ancient names, primitive, transition and 

 secondary, have all been objected to, and have been abandoned by many of the European 

 writers. In consequence of this, others have been proposed as substitutes, and have been 

 adopted in part ; but the proposed names are about as objectionable as the old ones, and 

 hence much hesitancy has been manifested in their adoption. Without attempting to decide 

 which nomenclature is best, I shall use that which the public is most familiar with. 



The word primary is a term whose meaning is well fixed in this country, being applied 

 to those masses which were consolidated before the creation of organic bodies. This term 

 then will be used to designate a class whose existence was anterior to that of organic beings. 

 It is true that some masses belonging to this class have been in a liquid or fused state since 

 the existence of organic bodies; still, so far as observation extends, the great mass or crust 

 of the earth is made up of granite, gneiss, mica slate, hornblende, serpentine and primary 

 limestone ; and doubtless these masses were consolidated anterior to the period spoken of. 



The word sedimentary is another term, the meaning of which cannot be misunderstood 

 or misapplied. It will be used to designate those masses which are really consolidated 

 sediments. It will often be used as synonimous with the word stratified, inasmuch as all 



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