IRREGULARITIES OF SURFACE. 527 



deposits. There is every gradation from the fresh rocks deep below the 

 surface, which do not well exhibit their structures and textures, through 

 those in which structures and textures are brought out with great clearness 

 by the process of weathering, to completely disintegrated and partly 

 decomposed rocks constituting the soils, in which no structures and textures 

 remain. The gradations of the process may be very well seen at many 

 localities where there are sufficiently deep cuts on the Piedmont Plateau of 

 the United States. 



SURFACES OF WEATHERING. 



Weathering is only one of the complex processes which have an 

 influence upon the surface features of the land. Transportation and ero- 

 sion are quite as important in this respect as weathering, or even more 

 important. But certain features of the surface are largely due to difference 

 in the rate of weathering. Wherever the rocks are heterogeneous they 

 differ in their power to resist weathering. The more resistant . rocks 

 commonly constitute the elevations, and the less resistant rocks the minor 

 features. This is true of both the major and the minor features. Illustra- 

 tions of this principle of various kinds will be mentioned. One of the most 

 common illustrations are elevations or ridges formed by dikes, which are 

 often more resistant than the surrounding rocks. In other cases dikes are 

 less resistant than the intruded rocks, and where the dikes once were may 

 be found deep, narrow-walled depressions or chasms, which may be occu- 

 pied by streams or, if along- the shore, by the water of the ocean or lake. 

 Basic dikes in the resistant gneisses furnish frequent instances of this. 

 Beautiful illustrations are seen on the coast of New England, especially 

 Maine. a 



Another interesting case of an irregular surface due to weathering is 

 produced by bowlders, of disintegration. The process of rock decay 

 follows along the planes of weakness, and especially along fractures, such 

 as joints. When a rock is cut by various sets of intersecting joints the 

 process of weathering goes on more rapidly at the corners of the rectan- 

 gular blocks than on the sides, thus producing round masses which 



« Lord, E. C. E., On the dikes in the vicinity of Portland, Me.: Am. Geologist, vol. 22, 1898, p. 

 338. Bascom, F., On some dikes in the vicinity of Johns Bay, Maine: Am. Geologist, vol. 23, 1S99, 

 pp. 275-280. 



