origins" a^d formatioi^ of soils. 7 



their combinations with potash, soda, and ammonia, act 

 energetically on carbonate of lime and on the oxides of 

 iron, and play a highly important part in the disintegra- 

 tion of various kinds of rocks. (5. ) The burrowing of 

 earth-worms and other earth- dwelling creatures brings 

 fresh particles to the surface, and admits the passage of 

 air and water to the subsoil. 



According to Darwin the solid rocks disintegrate even 

 in countries where it seldom rains and wiiere there is no 

 frost. And in conformity with the views lately advanced 

 by De Konnick, such disintegrations may be attributed 

 to the carbonic and nitric acids, together with the nitrates 

 of ammonia, which are dissolved in the dew. 



Eocks of various hardness weather at different rates. 

 Many of the slates and shales disintegrate rapidly. So do 

 granite and gneiss. Purely calcareous rocks, however, 

 weather quickest, the lime being dissolved out of them by 

 the rain-water. Over some limestone rocks this gives 

 rise to a lime- ^^ pan" in the subsoil. The dissolved lime 

 is carried down and often forms, with other materials 

 present, a layer as of concrete, the lime acting as a 

 cement. The same power which dissolves the carbonate 

 of lime in solid rocks, dissolves also that which may be 

 diffused through the soil. In chalk countries the rapid- 

 ity with which the lime wastes is shown by the residuum 

 of flints left on the surface; because though even these 

 flints gradually disintegrate, they do not decay so fast as 

 the chalk. In spite of much stone-picking, these flints 

 continue to make their appearance. 



The rocks which weather with most ease and rapidity 

 do not always exhibit most soil: often the reverse. A 

 pure limestone would exhibit hardly any weathered band 

 or soil, because the carbonic acid of the rain would al- 

 most at once dissolve and remove the particles it acts 

 upon. Even in the case of igneous rocks, the composi- 

 tion may be such that those which weather most rapidly 



