ATMOSPHERIC EFFECTS. 



according to their varying forms. Calcareous cliffs assume 

 cylindrical forms, fig. 75 c, which seem at a distance to resemble 

 fortifications. The faces of these cliffs are often worn into a 

 succession of terraces or steps, as in fig. 75 D. 



163. The effects of long- continued atmospheric action upon the 

 forms of solid rocks, are seen in many places on the surface of 

 continents, which the sea has not approached within historic times. 

 Certain granites are thus disintegrated so profoundly, as to reduce 

 the under-surface of the strata to a mass of gravel, forming holes 

 into which the pluvial water from ravines flows in all directions. 

 These rocks are also sometimes met with worn into rounded forms, 

 and piled one upon another, so as to he supported only at a 

 single point, forming what are popularly called rocking-stones, 

 fig. 76 B. Cases of this kind are especially presented in the case 

 of certain porphyritic granites. In mountains where granite is 

 decomposed with facility, it has been remarked that masses of 

 these rocks, more or less divided, present a sort of horizontal 

 layers, separated by vertical fissures, so as to reduce the whole to 

 a pile of irregular parallelepipeds, fig. 76, c. The angles and 

 edges being often worn away, the mass is reduced to a form 

 resembling a pile of cheeses, fig. 76 A. 



Fig. 76. 



164. Solid rocks are often traversed by vertical crevices filled 

 with matter more easily penetrable by water. In such cases, the 

 pluvial waters entering these crevices dissolve and ultimately 

 sweep away the matter which fills them, leaving the parts thus 

 separated without support. These ultimately fall to the foot of 

 the cliff (fig. 77). 



165. When waters bathe the foot of steep cliffs, they have a 

 tendency to dissolve and decompose their lower strata, leaving 



119 



