4 ATMOSPHERIC AGENCIES. 



is present, the products of vegetable decomposition, especially ammo- 

 nia and humus acids* 



Atmospheric agencies graduate so insensibly into aqueous agencies 

 that it is difficult to define their limits. "Water, holding in solution 

 carbonic acid and oxygen, may exist as invisible vapor ; or, partially 

 condensed as fogs ; or, completely condensed, as rain falling upon and 

 percolating the earth. In all these forms its chemical action is the 

 same, and, therefore, can not be separated and treated under different 

 classes ; and yet the same rain runs off from and erodes the surface of 

 the earth, comes out from the strata and forms springs, rivers, etc., all 

 of which naturally fall under aqueous agencies. We shall, therefore, 

 treat of the chemical effects of atmospheric water in the disintegration 

 of rocks, and the formation of soils, under the head of atmospheric 

 agencies ; and the mechanical effects of the same, in eroding the surface 

 and carrying away the soil thus formed, under the head of aqueous 

 agencies. In moist climates vegetation clothes and protects soil from 

 erosion, but favors decomposition of rocks and formation of soil. 



Atmospheric agencies are obscure in their operation, and, therefore, 

 imperfectly understood. Yet these are not less important than aqueous 

 agencies, since they are the necessary condition of the operation of the 

 latter. Unless rocks were first disintegrated into soils by the action 

 of the atmosphere, they would not be carried away and deposited as 

 sediments by the agency of water. These two agencies are, therefore, 

 of equal power and importance in geolog} 7 , but they differ very much in 

 the conspicuousness of their effects. Atmospheric agencies act almost 

 equally at all times and at all places, and their effects, at any one place 

 or time, are almost imperceptible. Aqueous agencies, on the contra- 

 ry, in their operation are occasional, and to a great extent local, and 

 their effects are, therefore, more striking and easily studied. Never- 

 theless, the aggregate effects of the former are equal to those of the 

 latter. 



Soils. — All soils (with the trifling exception of the thin stratum 

 of vegetable mold which covers the ground in certain localities) are 

 formed from the disintegration of rocks. Sometimes the soil is formed 

 in situ, and, therefore, rests on its parent rock. Sometimes it is re- 

 moved as fast as formed, and deposited at a distance more or less remote 

 from the parent rock. The evidence of this origin of soils is clearest 

 when the soil is formed in situ. In such cases it is often easy to trace 

 every stage of gradation between perfect rock and perfect soil. This 

 is well seen in railroad cuttings, and in wells in the gneissic or so-called 

 'primary region of our southern Atlantic slope. On examining such a 



* Alexis Julien, Proceedings of the American Association for the Advancement of 

 Science, vol. xxviii, p. 311 ct scq. 



