OF THE MORE DISTURBED ZONES OF THE EARTH'S CRUST. 439 



Progressive Flattening down of the Flexures. 



A third feature of gradation shows itself in the progressive sinking or flatten- 

 ing down of the successive individual flexures, until these finally pass into hori- 

 zontality. These three types of form in the waves, as respects their expansion, 

 their increase of relative distance or amplitude, and their declining height, are 

 conspicuously discernible, wherever we cross the great Appalachian chain of the 

 United States, by any section, in a direction from south-east to north-west. An 

 inspection of the engraved sections illustrating our paper on the physical structure 

 of the Appalachians,* or an examination of the more numerous similar diagrams 

 explanatory of the geological surveys of New Jersey, Pennsylvania, and Virginia, 

 will amply avouch for the correctness of this generalization. It is further borne 

 out in the published reports of the Government Survey of Canada, where the pli- 

 cated structure of the green mountain range of Lower Canada, along all its south- 

 eastern border, and the universality of the south-eastward dip of the folded strata, 

 — in other words, of the dip of the axis planes, — is very distinctly set forth by Sir 

 William Logan. 



Not only does the entire chain in its breadth exhibit a general gradation in the 

 several features here described, but each of its great component groups of flexures, 

 presents the same progressive opening, recession, and flattening down of its waves 

 in the same uniform north-west direction. 



Similar phenomena of gradation will, we feel assured, disclose themselves in 

 any section made from the Taurus range, north-westward through the Rhenish 

 Provinces and Belgium, where, on the one side, the more ancient and much meta- 

 morphosed strata at the base of the Palaeozoic system, according to the observa- 

 tions of Murchison and Sedgwick, present much reversal of the dip, and where 

 one and the same dip, namely, to the south-south-east, is continued with very few 

 exceptions across a belt of 50 miles ; whereas, on the opposite or northern side 

 of the zone, as is well shown in the beautiful sections of M. Dumont, the flexures 

 of the Belgian coal-fields are of the normal type, and much more open and 

 dilated. 



Nowhere perhaps in Europe are these gradations in the undulations of strata 

 more beautifully exposed than on the flanks of the Alps. Deep in, towards its 

 higher central igneous chains, the plication of its stratified rocks is excessive, and 

 the inclination of the axis planes remarkably low ; but advancing outwards, the 

 waves gradually lift their crests, throw forward their inverted sides, and assume 

 that type of flexure which we have called the normal one ; while, at the outward 

 base of the mountains just before these undulations are concealed by the over- 

 lapping tertiaries of the plains of Switzerland, and of Northern Italy respectively, 



* Transactions of the Association of American Geologists. 

 VOL. XXL PART III. 6 C 



