PRINCIPLES OF ROCK WEATHERING 707 



rhyolites in the Yellowstone National Park. Nevertheless, it is 

 to the exceedingly slow process of superficial weathering that we 

 owe a very large share of the apparent rock decomposition and 

 incidental soil formation/ 



Were it possible it might be well in this discussion to describe 

 each of the involved processes in detail, both in relation to its 

 mode of operation and the results produced. From the 

 fact however, that anyone, either physical or chemical, rarely goes 

 on alone, it is thought best to treat the subject as below, and 

 describe in more or less detail the action of ( i ) the atmosphere, 

 (2) of water in both the solid and liquid form, and (3) that of 

 plant and animal life, finally considering the combined action of 

 all these forces, as manifested on the various types of rock which 

 go to make up the earth's crust. 



{a) ACTION OF THE ATMOSPHERE. 



Pure dry air under constant conditions of heat or cold, can 

 have, but little effect upon rock-masses either in producing 

 physical or chemical changes. Aided, however, by moisture and 

 temperature variations, it becomes a powerful agent for disin- 

 tegration as well as for transportation. 



In its normal state atmospheric air, as is well known, is a 

 mechanical admixture of four volumes of nitrogen to one of 



' The reader must keep clearly in mind the distinction between the words altera- 

 tion (Ger. Umwandlung), and decomposition (Ger. Verwitterung or Zersetzung) as here 

 used. The one is a more or less deep-seated chemical and molecular process through 

 which a rock may undergo a complete change so far as its mineralogical or lithological 

 nature is concerned while yet retaining its geological identity, as where augite becomes 

 altered to uralitic hornblende, or an eruptive olivine rock (peridotite) becomes altered 

 into serpentine. The second is a wholly superficial change brought about through 

 external agencies and resulting in a more or less complete destruction of the original 

 compounds, loss of material and general breaking down of the mass as a geological 

 body, as when granitic rocks decompose to the condition of quartz sand and kaolin, 

 with the separation of free calcium and alkaline carbonates and oxids of iron and 

 manganese. The line of distinction to be sure cannot in nature be always sharply 

 drawn, and indeed alteration is often but a preliminary to decomposition, though this 

 is by no means universally true, as is shown by the superior resisting power of certain 

 trappean rocks in which the pyroxenic and feldspathic constituents have altered into 

 hard, tough aggregates of free quartz, chlorite and epidote. 



