496 OF THE PHYSICAL STRUCTURE 



prevailing in a very remarkable line of fault, which extends, with 

 but few interruptions, along the western margin of the Great Val- 

 ley of Virginia, throughout the chief part of its length. The ridge, 

 which bounds this valley on the northwest, and which, as we 

 pursue it southwestward, assumes successively the names of 

 Little North mountain. North mountain, and Brushy ridge, marks , 

 the position of this extraordinary dislocation. With the excep- 

 tion of several intervening spaces, some distance south of the 

 James river, in which the normal, or northwestern dip of the 

 rocks in this ridge is in the main retained, its sti'ata assume an 

 inverted attitude, the gi'eat lower Appalachian limestone of the 

 valley, lying on the slates of the next superior gi'oup, and these, in 

 turn, resting with a southeast dip on the white sandstone, while 

 the adjoining formations of a still higher position in the series, are 

 either partially or entirely sw^allowed in the fissure. The 

 sandstone itself, which, throughout a part of the State, gives prom- 

 inence to the ridge, and the slates intervening between it and the 

 limestone, are both more or less ingulfed ; and, in some parts of 

 the line, the whole mass of the mountain has disappeared ; so that 

 the observer may cross, by a single stride, from the very ancient 

 limestone of the valley, to beds but little lower in the series than 

 the carboniferous limestone. Still further along the line, the for- 

 mations thus lost are seen partially rising again into view, the 

 white sandstone (F. IV) first showing itself in insulated knobs or 

 patches, and afterwards in a continuous, low, and irregular ridge, 

 in which some of the other missing groups also reappear. Be- 

 tween a point a few miles south of the intersection of the James 

 river with this ridge, and the neighborhood of Abingdon, near 

 the Tennessee line, this fault assmnes a more uniform, though, 

 perhaps, a stiD more extraordinary character. At the passage of 

 the river, the massive range of the North mountain presents no 

 other indications of this line of fault than a partial inversion of 

 the thick beds of sandstone, of which it principally consists, and 

 an .entire overthrow and partial burial of the slates which flank it 

 on the southeast. But a few miles further towards the southwest, 

 the whole of this enormous mountain mass sinks from view, 

 excepting an isolated knob here or there, of the harder rocks, 



