THE WORK OF BUXXTXG WATER. 



143 



Dikes of igneous rock, harder than the beds which they intersect, hke- 

 -^nse become ridges after the degradation of their siirroimclings. The 

 phigs of okl volcanic vents and other igneous intrusions of hmited 

 area often constitute conspicuous hills or mountains after erosion has 

 removed their less resistant surroundings (Fig. 124). Inequalities of 

 hardness are therefore responsible for many hills and ridges. In the 

 isolation of the hills and ridges picturesque coves are developed, 

 where the attitude and chstribution of the weak and strong rocks are 

 propitious. The bottoms of the coves are located on the weak rocks, 

 and above them rise the precipitous slopes of the resistant ones. Round 

 valley (Fig. 1, PI. XVII, High Bridge, X. J., quadrangle, U. S. Geol. 





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Fig. 124. — Matteo tepee, Wyo. Mass of igneous rock exposed by erosion, and pre- 

 served because of its superior resistance. (Detroit Photo. Co.) 



Sur^-.) and the coves about the head of Hiawassee River (Dahlonega, 

 Ga., quadrangle) are examples. 



Ridges and hills resulting from the unequal degradation of un- 

 equally resistant terranes are not equally prominent at all stages 

 in an erosion cycle. In early youth the material surrounding the 

 hard bodies of rock has uoi been removed; in Nearly maturity con- 

 siderable portions of their surroundings still remain about them; but in 

 late maturity or early old a§e the outcropping masses of hard rock 



