512 OF THE PHYSICAL STRUCTURE 



disturbed mountainous districts, we conceive to have originated in 

 the following manner. We assume, that in every region, where 

 a system of flexm'es prevails, the crust previously rested on a 

 widely extended surface of fluid lava. Let it be supposed, that 

 subterranean causes competent to produce the result, such, for 

 example, as the accumulation of a vast body of elastic vapors 

 and gases, subjected the disturbed portion of Ihe belt to an ex- 

 cessive 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 withdrawn, this would rise along the whole line of fissm'e in the 

 manner of an enormous billow, and suddenly lift with it the overly- 

 ing flexible crust. Gravity, now operating on the disturbed lava 

 mass, would engender a violent undulation 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 

 con-esponding billowy motion to the overlying strata. Simulta- 

 neously 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 ad- 

 vanced side of each wave, precisely as the wind, acting on the 

 billows of the ocean, forces foi-ward their crests, and imparts a 

 steeper slope to their leeward sides. A repetition of these forces, 

 by augmenting the inclination on the front of every 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 horizontal force, but by 

 the production in this distiict of many lesser groups of waves, in- 

 terposed between the larger ones, and not endowed with sufficient 

 momentum to reach the remoter sides of the belt. To this inter- 

 polation we attribute, in pai*t, the crowded condition of the axes 



