STRUCTURAL CURVES AND THEIR PRODUCTION. 135 



istics of structure which prevail in the midst of the infinite variety of 

 relief forms. 



Weather Curve and water Curves. 



Weather Carre. — The weathering of structural blocks reduces their 

 salient angles, which are attacked from both of the adjacent faces at once. 

 At the point x, figure 1, the disintegrating forces act with twice as great 

 intensity as at b, since the attack conies from two directions. The effects 

 are more than twice as great at x, because the products of decay are 

 quickly removed, exposing fresh surfaces to the attack, while at b they 

 remain to cover and protect the subjacent beds. Thus the structural 

 block m n o p is rounded oil' by weathering. The new outline a b c is 

 composite. The portion <1 b e is ;i weather curve, convex upward. If 

 weathering alone, without the aid of flowing water, has been concerned 

 in the sculpturing process, the talus slopes a d and e c will be structural 

 planes, not curves. The structural angle e c p will he determined by the 





am j> c 



FtiiUHE 1. —Weather Curve. 



resting angle of the materials composing the talus, and that again will 

 depend upon the size and form of the particles ; hut in humid regions 

 the talus slopes will he quickly molded into water curves, as hereaiter 

 described. The resulting form <i b c will lie a rounded rock, a smooth 

 knob, or a round-topped hill or mountain, according as the original block 

 was measured by inches or leagues. 



Convexity of weather Curve. — The upward convexity of weather curves 

 may be deduced also from the law Hint declivities vary directly as hard- 

 ness. If we suppose the lowest beds composing a structural block to he 

 very soft and the hardness to increase upward by regular gradations, ;i 

 concave slope would result. Such a curve may actually be formed, hut 

 it would have to be ascribed to structure, not to weathering. Weather 

 curves are often interrupted and modified by such structural accidents. 

 But if the opposite conditions prevail — that is to say, if the soft bed is 

 above and the hardness increases regularly downward -the law just 

 enunciated will yield a slope of convex curvature. Now, whatever may 



