238 



THE GEOLOGIST. 



or successively along parallel lines could only produce a 

 series of symmetrical flexures, whilst tangential-pressure, 

 unaccompanied by a vertical force, would result in irregular 

 contortions dependent on local irregularities in the amount 

 of resistance. The alternate upward and downward move- 

 ments necessary 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 direc- 

 tion 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 move- 

 vements, or by ^^a merely upward pressure, unaccompa- 

 nied with pulsations on the surface of a fluid ; and if this 

 force be feeble and oft repeated, it is difiicult to understand 

 how it could return always to the same lines until they be- 

 came conspicuous flexures." The authors suppose the 

 strata of the region in question to have been subjected to 

 excessive upward tension arismg from the expansion of 

 molten matter and gaseous vapours ; the tension would at 

 length be relieved by many parallel fissures formed in suc- 

 cession, through which much elastic vapour 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 communi- 

 cate 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 tan- 

 gential 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 bo- 

 dies of elastic vapour, might open and close again superfi- 



