AMERICAN GEOLOGY DECADE OF 1840-1849. 371 



the strata at a distance of about 150 miles from the chain of the Blue 

 Ridge or South Mountain. 



This work, so far as it relates to the structure of the chain, has 

 been improved upon in recent times only by the discovery of enor- 

 mous overthrust faults in the southwestern portions. Their ideas 

 regarding the origin of the chain have not, however, so successfully 

 withstood the work of the modern physicists. They assumed that 

 this difference in the dips was due to a combined undulatory and 

 tangential movement of the earth's crust, which was propagated 

 from the southeast toward the northwest; that is, they regarded the 

 various ranges composing the chain as actually stiffened waves or 

 billows of crustal matter comparable to the waves of less viscous mat- 

 ter, like water or lava. Their views may be best understood by direct 

 quotation: 



We assume that in every region where a system of flexures prevails the crust orig- 

 inally rested on a widely extended surface of fluid lava. Let it he supposed that 

 Subterranean causes competent to produce the result, such, for example, as the accu- 

 mulation of a vast body of elastic vapors and gases, subjected the disturbed portion 

 of the belt to an excessive upward tension, causing it to give way at successive times 

 in a series of long parallel rents. By the sudden and explosive escape of the gaseous 

 matter, the prodigious pressure, previously exerted on the surface of the fluid within, 

 being instantly withdrawn, this would rise along the whole line of fissure in the 

 manner of an enormous billow and suddenly lift with it the overlying flexible crust. 

 Gravity, now operating on the disturbed lava mass, would engender a violent undu- 

 lation of its whole contiguous surface, so that wave would succeed wave in regular 

 and parallel order, flattening and expanding as they advanced, and imparting a cor- 

 responding billowy motion to the overlying strata. Simultaneously with each epoch 

 of oscillation, while the whole crust was thus thrown into parallel flexures, we suppose 

 the undulating tract to have been shoved bodily forward and secured in its new- 

 position by the permanent intrusion into the rent and dislocated region behind of 

 the liquid matter injected by the same forces that gave origin to the waves. This 

 forward thrust, operating upon the flexures formed by the waves, would steepen 

 the advanced side of each wave precisely as the wind, acting on the billows of the 

 ocean, forces forward their crests and imparts a steeper slope to their leeward sides. 

 A repetition of these forces by augmenting the inclination on the front of even- 

 wave would result finally in the folded structure, with inversion in all the parts of 

 the belt adjacent to the region of principal disturbance. Here an increased amount 

 of plication would be caused, not only by the superior violence of the forward hori- 

 zontal force, but by the production in this district of many lesser groups of waves 

 interposed between the larger ones and not endowed with sufficient momentum to 

 reach the remoter sides of the belt. To this interpolation we attribute, in part, the 

 crowded condition of the axes on the side of the undulated district which borders 

 the region where the rents and dikes occur, and to it we trace the far greater variety 

 which there occurs in the size of the flexures. 



The date of the Appalachian uplift they put subsequent to the for- 

 mation of the Coal Measures, as being the final paroxysmal movement 

 which terminated "in that stupendous train of actions which lifted the 

 whole Appalachian chain from the bed of the ancient sea." 



