EROSIVE AGENTS. 83 



from the same cause, is very common ; as in the case of iron and manganese, 

 which are almost universally present in the rocks and the soils. 



Nitrogen seems rarely to operate directly upon the rocks : but when converted 

 into nitric acid and ammonia, as it sometimes is, these compounds act with much 

 energy as disintegrating agents. 



Perhaps carbonic acid is the most efficient of all agents in the work of erosion. 

 But as it acts chiefly when dissolved in water, I reserve details to the next head. 

 And as to aqueous vapor in the atmosphere, its effects upon the rocks, are not so 

 marked as to be easily described, although doubtless it assists the other constituents 

 of the air in this work. 



2. Water. 



Water acts upon rocks and minerals in three modes, in all of which it is 

 energetic. 



1. As a medium for other decomposing agents. — These it dissolves, and thus 

 enables them to act upon the rocks. Carbonic acid should stand at the head of 

 this list. It seems to be the only acid, with a few rare exceptions, that exists in 

 the water, which penetrates the rocks, and is able to decompose the silicates of 

 alkalies, the alkaline earths, and protoxides of iron and manganese, at ordinary 

 temperatures. The alkaline carbonates when formed, will decompose solutions of 

 sulphate of lime, or manganese, and the chlorides of calcium and magnesium. 

 Chemical changes thus begun, others will follow in a wider range, all commencing 

 with carbonic acid. 



Bicarbonate of lime is another agent widely diffused and productive of extensive 

 changes : such, for instance, as the formation of carbonate of lime, the most 

 abundant of the salts formed in the earth's crust. 



The alkaline carbonates are not so generally found in natural waters : but when- 

 ever present, as the result of other agents, they effect important changes, such as 

 prepare the way often for erosions by mechanical agencies. 



2. Water, alone, dissolves not a few of the ingredients of rocks. — The process is 

 much slower generally than when aided by carbonic acid. Nevertheless, pure water 

 will dissolve most of the refractory minerals. Hot water will do it most rapidly : 

 but cold water will do essentially the same, if sufficient time be given. Professors 

 W. B. and R. E. Rogers, in this way dissolved portions of more than thirty 

 rocks and minerals, seemingly the most unyielding, such as feldspar, mica, augite, 

 tourmaline, hornblende, chalcedony, epidote, talc, serpentine, obsidian, lava, green- 

 stone, gneiss, and hornblende slate. It has been probably by means of water 

 chiefly, that the various pseudomorphous processes, which we find to have gone on 

 so extensively in the mineral kingdom, have been accomplished. So great have 

 these changes been, that an able writer (Bischof ) says, that " strictly speaking, we 

 do not know with regard to any single mineral, whether it is still in its original 

 condition, or has been more or less altered." 



The power of water to penetrate rocks and minerals should be stated in this 

 connection. It not only makes its way into the cracks, fissures, and planes of 



