OF THE MORE DISTURBED ZONES OF THE EARTH S CRUST. 441 



chian zone, especially in those where the chain is convex to the south-east, and in 

 the straight sections of South-western Virginia and Eastern Tennessee. But I 

 am persuaded, from the descriptions of geologists, and from my own observations 

 that the fractures of this class are equally numerous in the Jura Mountains, in 

 the Alps, in the district of the Ardennes, in Belgium, and in the mountain chains 

 of Scotland. A leading feature of these great fractures is their parallelism to the 

 main anticlinal axes, or lines of folding of the chains to which they belong. They 

 are, in fact, only flexures of the more compressed type, which have snapped and 

 given way in the act of curving, or during the pulsation of the crust. They 

 coincide, in the great majority of instances, neither with the anticlinal nor the 

 synclinal axis planes of the waves or folds, but with the steep or inverted sides of 

 the flexures, and almost never occur on their gentler slopes. This curious and in- 

 structive fact may be well seen in the Appalachians of Pennsylvania and Virginia, 

 by tracing longitudinally any one of their great faults from its origin on the steep 

 flank of an anticlinal wave along the base of its broken crest to where the anti- 

 clinal form is resumed again. The following brief description, from our memoir 

 on the Physical Structure of the Appalachians, taken from the Transactions of 

 the American Association, will show the general phases through which these 

 fractures pass : — 



" From a rapidly steepening north-west dip, the north-western branch of the 

 arch (or flank of the wave) passes through the vertical position to an inverted or 

 south-eastern dip, and at this stage of the folding the fault generally commences. 



" It begins with the disappearance of one of the groups of softer strata lying 

 immediately to the north-west of the more massive beds, which form the irre- 

 gular summit of the anticlinal belt or ridge. The dislocation increases as we fol- 

 low it longitudinally, group after group of these overlying rocks disappearing from 

 the surface, until, in many of the more prolonged faults, the lower limestone forma- 

 tion (Cambrian or Lower Silurian) is brought for a great distance, with a moderate 

 south-easterly dip, directly upon the Carboniferous formations. In these stupen- 

 dous fractures, of which several instances occur in South-western Virginia, the 

 thickness of the strata ingulfed cannot be less, in some cases, than 7000 or 8000 

 feet." 



One of these enormous faults in South-western Virginia has a length of 

 more than 80 miles, and is almost perfectly straight. It follows the south-eastern 

 slope of Brushy Mountain, from the head of Catawba Creek to the vicinity of the 

 court-house of Smyth county, engulfing all the strata of the south-eastern half of 

 a synclinal basin, of which the Brushy Mountain remains as the other half. 

 Where the dislocation attains its maximum intensity, or shows the greatest dis- 

 placement of the strata, the lower formation, — the Auroral Appalachian limestone, 

 the equivalent of the Festiniog group of England, — of one side of the fissure, 

 rests in an inverted attitude, with a gentle south-east dip, directly on the south-east 



