842 



PRINCIPLES OF STRATIGRAPHY 



the center is still composed of a soft stratum, a central lowland will 

 remain as in the case of the Weald of southeastern luigland. If, 

 however, erosion goes on until the underlying crystallines are ex- 

 posed a central mountainous area will remain as in the case of the 

 Black Hills. (Fig. 214.) 



In outcrop, an eroded dome will show the strata in a series of 

 concentric rings, the oldest at the center and the youngest outer- 



— -^-^i/ 



Fig. 214. Stereogram of the Black Hills dome, showing the. mountainous 

 center formed by the resistant crystallines, and the rimming 

 hog-backs and valleys. (After Davis.) 



most. As in the case of the cuesta and the monocline, the ultimate 

 result of erosion of such a dome is the obliteration of the ridges, 

 and the reduction of the dome as a whole to peneplain condition. 

 When that has occurred all the strata involved will be beveled off 

 toward the center of the dome, their lower edges projecting farthest 

 up onto the dome. ( Fig. 215, A.) A structure of this kind is not 

 infrequently mistaken for a marginal thinning of strata on the shore 



215- 



Diagrams illustrating the thinning of strata under cover toward the 

 center of a dome : A, by erosion ; B, by overlap. 



of an island. In this case, however, the strata should overlap each 

 other, and the higher portions reach farthest onto the dome. ( Fig. 

 215, B.) The thin overlapping edges, moreover, should be of a clas- 

 tic character, and composed in part of material derived from the 

 shore of the island, while, in the case of the eroded dome, the strata 

 on both sides, being part of a formerly continuous whole, should be 

 of the same character, and show no shore features on the thin 

 edge. Furthermore, strata deposited in this area, subsequently to 



