OF THE APPALACHIAN CHAIN. 485 



eral results of our observations, will be found several actual sec- 

 tions, comprising the principal details of structure and topography- 

 observed in different parts of the chain, from New Jersey to east- 

 ern Tennessee. These cross the belt at nearly equal intervals, 

 and have been selected from a number, all of which equally ex- 

 hibit the general conditions of structure above described. 



To assist in conveying clear conceptions of the diversified and 

 sometimes complicated modes of sti'ucture, occasioned by the 

 flexures and foldings of the strata, we deem it important to intro- 

 duce here two or three new descriptive terms, which seem called 

 for by the necessity of possessing a phraseology adapted to the 

 relationships of the strata about to be detailed. Using the terms 

 anticlinal and synclinal in their commonly accepted sense, we 

 propose to apply the phrases anticlinal or synclinal mountain or 

 range, to designate ridges formed respectively by a convex and 

 concave flexure of the strata. Every flexure, of such degree as to 

 fall short of producing an inversion of the rocks on the northwest- 

 ern side of the anticlinal, and the southeastern side of the syncli- 

 nal bends, we shall call a normal flexure ; and the dips correspond- 

 ing to such flexures, as exhibited in transverse sections, we shall 

 denominate normal dips. While the phrases, anticlinal dip, and 

 synclinal dip, sufficiently express the directions of the beds, due 

 to the concave and convex flexures, we propose the term mono- 

 clinal, to signify a sameness in the direction of the dip, and shall 

 term a mountain or valley, in which such sameness prevails, a 

 monoclinal mountain, or monoclinal yaWej. As briefly expressive 

 of the whole concave and convex flexure, we propose to use the 

 terms arch and trovgh. 



Conceiving a plane to be extended through the apex or most 

 incurved part of each of the concentric flexures in an anticlinal or 

 synclinal bend, so as to occupy a medial position betsveen the 

 two branches of the curve, we propose to call this plane the axis- 

 plane. Where the flexure is perfectly symmetrical on both sides 

 of the plane, and the dip on the one side, therefore, equal to that 

 on the other, it is evident, that the axis-plane will have a vertical 

 position. In the Appalachian region, however, and, as we believe, 

 in nearly all other disturbed chains, where the phenomena of 



