412 Theories of the Formation of Mountains, 



its upheaval that the inexperienced observer may well fail to detect 

 many indications of the systematic regularity represented in the 

 above figure. Yet there remain sufficient of the original foldings 

 of the strata to enable the physical geologist to demonstrate that 

 Roger's elucidation of the structure is upon the whole correct. 



The following are the principal theories of the origin of the 

 flexures of the Appalachian Mountains. 



Prof. Roger's Theory. 



"The wave-like structure of the Appalachians and other undulated 

 zones has been attributed by the author and his brother, Prof. W. B. 

 RogervS, in their communications to the American Association in 

 1842, and to the British Association in the same year, to an 

 actual undulation of the supposed flexible crust of the earth, 

 exerted in parallel lines, and propagated in the manner of a hori- 

 zontal pulsation from the liquid interior of the globe. We sup- 

 pose the strata of such a region to have been subjected to exces- 

 sive upward tension, arising from the expansion of molten matter 

 and gaseous vapours, the tension relieved by linear fissure*, through 

 which much elastic vapour escaped, the sudden release of pres- 

 sure adjacent to the lines of fracture producing violent pulsations 

 on the surface of the liquid below. This oscillating movement in 

 the fluid mass beneath would communicate a series of temporary 

 flexures to the overlying crust, and these flexures would be ren- 

 dered permanent (or keyed into the forms they present) by the 

 intrusion of molten matter. If during this oscill/ition we conceive 

 the whole heaving tract to have been shoved (or floated) bodily 

 forward in the direction of the advancing waves, the union of this 

 tangential with the vertical wave like movement will explain the 

 peculiar steepening of the front side of each flexure, while a repe- 

 tition of similar operations will account for the folding under or 

 inversion, visible in the more compressed districts. We think 

 that no purely upward or vertical forces exerted either simultan- 

 eously or successively along parallel lines, could produce a series 

 of symmetrical flexures, and that a tangential pressure, unac- 

 companied by a vertical force, would result only in an impercep- 

 tible bulging of the whole region, or an irregular plication depen- 

 dent on local inequalities in the amount of resistance. The 

 alternate upward and downward movement necessary to enable a 

 tangential force to bend the strata into a series of regular parallel 

 subsiding flexures has been we conceive, of the nature of a 



