34 PRINCIPLES OF STRATIGRAPHY 



hydration, etc., according" to the state of the atmosphere. Where 

 httle moisture exists in the air, as in arid regions, the chief process 

 hy which the destruction of rock masses is effected is disintegration, 

 supplemented, of course, by the mechanical work of the wind. 



Branner (10:281) has observed that the mica of Brazilian 

 gneisses is an element of weakness, tending, both on account of its 

 low conductivity and its form, to develop crevices along which the 

 rocks exfoliate more readily. 



Frost JVork. Water, on freezing, expands one-tenth of its vol- 

 ume, and so becomes a powerful agent in destroying rock masses. 

 The expansion of water in fissures on freezing is capable of prying 

 off huge masses of rock, and in some cases much of the talus at the 

 foot of the cliff is to be referred to frost action. Rock surfaces may 

 by radiation cool below the freezing point (0° C. or 32° F.), so 

 that moisture coming in contact v.'ith them freezes at once. This is 

 often seen in high altitudes where moisture-laden winds strike a cold 

 rock wall or cairn, with the result that the moisture is precipitated 

 as ice. An interesting case of freezing in contact with cold rocks 

 has been observed on the coast of Newfoundland, where the lime- 

 stone cools more rapidly at the beginning of the winter than the sea- 

 water, which penetrates all the fissures at high tide, freezing there 

 in contact with the cold rock and shattering the mass during the re- 

 treat of the tide. The constant repetition of this phenomenon re- 

 sults in covering the beach with freshly broken, angular fragments. 

 (Thoulet, Walther-103 :55p.) 



The creep of the soil on sloping surfaces has also been referred 

 to frost action, the water in the soil freezing and converting the 

 mass into an "earth glacier" (Kerr-54:?_/5). The creejiing down- 

 hill of this mass results in the bending over of the ends of the ver- 

 tical or steeply inclined strata. 



CHEMICAL WORK OF THE ATMOSPHERE. 



The chemical activities of the atmosphere penetrate the upper 

 layers of the lithosphere to the level of the ground w^ater. This 

 averages perhaps from 30 to 50 meters below the surface, though in 

 many regions it is at the surface, while again, in arid regions it may 

 be so much as 1,000 meters or more beneath the surface. The belt 

 thus produced, between the average level of the ground water and 

 the surface, is the belt of denwrphism, or in general the Belt of 

 Weathering. It is here that the chemical work of the atmosphere 

 is most marked in destroying the rocks, this work consisting, so far 



