494 OF THE PHYSICAL STRUCTURE 



plane, that sometimes passes into a fault. This inversion is 

 strikingly displayed along the southeastern base and slope of the 

 synclinal mountain, called Buck-horn ridge, adjoining the axis 

 on the northwest, where the strata of this side of the mountain are 

 folded over so as to lie in almost parallel posture with the corres- 

 ponding rocks on the opposite or northwestern side of the trough. 



3rd. Flexures broken^ or jJcissing- into faults. A feature of 

 frequent occurrence in certain portions of the Appalachian belt, 

 is the passage of an inverted or folded flexure into a fault. These 

 dislocations, preserving the general direction of the anticlinal axes, 

 out of which they grow, are usually prolonged to a great distance, 

 having, in some instances, — for example, in southwestern Vir- 

 ginia, — a length of about one hundi'ed miles. These lines of 

 fault occur in all cases, along the northwestern side of the anti- 

 clinal, or the southeastern side of the synclinal axis, and never in 

 the opposite situation. This cmious and instructive fact is best 

 seen by tracing, longitudinally, some of the principal anticlinal 

 axes of Pennsylvania and Virginia. From a rapidly steepening 

 northwestern dip, the northwestern branch of the arch passes 

 through the vertical position, into an inverted or southeastern dip ; 

 and at this stage of the folding, the fault generally commences. 

 It begins with the disappearance of one of the gi'oups of softer 

 strata, lying immediately to the northwest of the more massive 

 beds, which form the northwestern summit of the anticlinal belt. 

 The dislocation increases as we follow it longitudinally, group 

 after group of these overlying rocks disappearing from the sur- 

 face, until, in many of the more prolonged faults, the lower lime- 

 stone is brought, for a great distance, with a moderate southeast- 

 erly dip, directly upon the carboniferous formations. In these 

 stupendovTS fractures, of which several instances occur in south- 

 western Virginia, the cai-boniferous limestone being brought into 

 close proximity to the great lower Appalachian limestone, a portion 

 of which, even, is occasionally buried, the thickness of the sti'ata 

 ingulfed cannot be less than seven thousand or eight thousand 

 feet. 



The position of the strala along some of these extraordinary 

 dislocations may be seen in the Sections C, D, E, (Pis. XX, XXI,) 



