498 OF THE PHYSICAL STRUCTURE 



dynamics into which the whole subject of flexures must evident- 

 ly lead us. The limits of the present memoir preclude a detailed 

 description of each group of axes, contained even in the States of 

 Virginia, Pennsylvania, and New Jersey, where we have princi- 

 pally explored them, and altogether forbid any attempt to apply 

 our theory of flexures to an explanation of the local features, dis- 

 tinctive of each gi-oup. We shall, therefore, content ourselves 

 with describing two or three of these collections of axes, more for 

 the purpose of proving our present general statement, that the 

 axes do thus assort themselves, than with a view to discuss the 

 secondary causes connected with their peculiarities. 



The gi-eat divisions, into whjch the entu-e chain naturaUy di- 

 vides itself, are alone abundanfly significant of this essential ten- 

 dency of the axes to form groups. For, upon a general view of 

 the whole chain, each of the nine extensive belts, into which we 

 have divided it, becomes one comprehensive group, in wiiich all 

 the axes display certain common characteristics of straightness or 

 curvature, as the case may be. Lest, however, it should be sup- 

 posed, that this gi'ouping of the flexures is only to be recognized 

 when we embrace very extensive subdivisions of the chain, we 

 shall refer to smaller tracts, and show, that axes of all dimensions 

 thus associate themselves. An excellent instance of a gi-oup is 

 to be seen in a district composed of the northern half of Mifflin, 

 and the southeastern half of Centre counties, in Pennsylvania. 

 The axes which belong to the general convex system of the Ju- 

 niata accord remarkably, in all their essential features. They are 

 either of the wiormal type, with steep northwestern dips, or they 

 have the northwestern part of the arch slightly inverted. They 

 are almost exactly parallel, curving a little in obedience to the 

 general sweep of the chain, while they are singularly equidistant 

 from each other. As each flexure possesses nearly the same trans- 

 verse form and dimensions, they bestow a strikingly regidar and 

 symmetrical topography on the wliole region, the great lower 

 Appalachian limestone and slate groups rising to the surface in 

 a series of long and parallel anticlinal valleys, while the overlying 

 sandstones compose so many intervening, steep, straight, and reg- 

 ular synclinal ridges. 



