652 



DYNAMICAL GEOLOGY. 



Figs. 1081, 1082, represent the common character of such hills. 1 A number are shown 

 in Fig. 1080 ; in the Colorado region, they have been called Mesas, from the Spanish for 

 table. In some parts of the Rocky Mountain slopes, the thick gravel deposits are cov- 

 ered with streams of lava of great thickness; and table mountains are common in such 

 regions (Fig. 1120, p. 741). 



Elevations thus left prominent, after denudation around, have been called hills, or 

 mountains, of circumdenudation. Figs. 1083, 1084, are other examples. 



When the beds are inclined between 5° and 30°, and are alike in hardness, there is a 

 tendency to make hills with a long back slope and bold front ; but, with a much larger 

 dip, the rocks, if hard, often outcrop in naked ledges. 



When the dipping strata are of unequal hardness, and lie in folds, there is a wide 

 diversity in the results on the features of elevations. 



Figs. 1083, 1084, represent the effects from the erosion of a synclinal elevation consist- 

 ing of alternations of hard and soft strata. The protection of the softer beds by the 

 harder is well shown. This is still further exhibited in Figs. 1085-1088. 



Anticlinal strata give rise to another series of forms, in part the reverse of the pre- 

 ceding, and equally varied. Figs. 1089-1092 represent some of the simpler cases. 



1083 



Figs. 1083-1088. 



When the back of an anticlinal mountain is divided (as in Figs. 1089-1092), the moun- 

 tain loses the anticlinal feature; and the parts are simply monodinal ridges. As the 



Figs. 1089-1092. 



1091 



JMk 



1092 



anticlinal, in the progress of its formation, is almost sure to have its back fractured, 

 from the strain on the bending rocks, the removal of the upper and central portion, 

 making a broad valley in its place, is the common fact. 



In Fig. 1092, the anticlinal character is distinct in the central portion, while lost in the 

 parts either side. To the right, in this figure, is shown a common effect of the protection 

 afforded to softer layers by even a vertical layer of hard rock: the vertical layer forms 

 the axis of a low ridge. 



The above are the simple results from the erosion of folded rocks, whatever the 

 agency concerned. They serve as a key to the complexities of features common through 

 a large part of the Appalachians and other regions of folded rocks, where synclinal and 

 anticlinal axes are in numberless complicated combinations, rendered doubly puzzling 

 by faults. See, further, pages 93-98, and beyond, page 786. 



The outlining of mountain-ridges and valleys has been sometimes 

 produced by subterranean forces, uplifting and fracturing the strata ; 



1 For Figs. 1081-1092, and the views they illustrate, the author is indebted to the 

 volume on Coal and its Topography, by Lesley. In a long chapter on " Topography 

 as a Science," this author has given the results of extensive personal observation. 



