Proceedings of the British Association. 361 



contortions dependent on local irregularities in the amount of re- 

 sistance. The alternate upward and downward movements ne- 

 cessary to enable the tangential force to bend the strata into a 

 series of flexures, are such " as would arise from a succession of 

 actual waves rolling in a given direction beneath the earth's 

 crust." The authors observe that it would be difficult to account 

 for the formation of grand yet simple flexures, by a repetition of 

 feeble tangential movements, or by " a merely upward pressure, 

 unaccompanied with pulsations on the surface of a fluid ; and if 

 this force be feeble and oft repeated, it is difficult to understand 

 how it could return always to the same lines until they became 

 conspicuous flexures." The authors suppose the strata of the 

 region in question to have been subjected to excessive upward 

 tension, arising from the expansion of molten matter and gaseous 

 vapors ; the tension would at length be relieved by many parallel 

 fissures formed in succession, through which much elastic vapor 

 would escape, and by thus removing the pressure adjacent to the 

 lines of fracture, produce violent pulsations on the surface of the 

 fluid below. This oscillatory movement would communicate a 

 series of temporary flexures to the overlying crust, which would 

 be rendered permanent by the intrusion of molten matter into 

 the fractured strata originating the tangential force by which the 

 flexures received their peculiar character before described. The 

 authors do not deem it essential to this explanation, that in the 

 production of axes of elevation, the strata should be permanently 

 fractured to the surface. Fissures sufficient for the escape of vast 

 bodies of elastic vapor, might open and close again superficially j 

 and the strata may often be supported in their new position by 

 subterranean injections not visible on the surface. 



Identity of the undulations which 'produced the axes, with the 

 wave-like motion of the Earth in Earthquakes. — The authors 

 suppose all earthquakes to consist in oscillations of the earth's 

 crust propagated with extreme rapidity; and they ascribe this 

 movement to a sudden change of vertical pressure on the surface 

 of an interior fluid mass, throwing it into wave-like undulations, 

 such as would produce permanent flexures in the strata if more 

 energetic, accompanied by the formation of dykes. The succes- 

 sive earthquakes of any region usually proceed from the same 

 quarter, and this must also have been the case with the move- 

 ments which gave rise to the parallelism of contiguous anticlinal 



Vol. xliv, No. 2. — Jan.-March, 1843. 46 



