Proceedings of the British Association. 569 



groups within the large divisions the parallelism is still more exact. 

 The axes vary in length from insignificant flexures to lines frequently 

 100 and sometimes 150 miles in length, and they deviate very little 

 from a rectilinear course, or, as the case may be, from a uniform rate 

 of curvature. Some of the longer curved axes exhibit a difference of 

 strike at their extremities of 50 in a distance of 90 miles, and the 

 rectilinear axes of different divisions vary in their line of direction as 

 much as 60°: As all the flexures were undoubtedly formed at one 

 period, the authors consider these facts at variance with M. Beaumont's 

 hypothesis, that dislocations of the same geological age are parallel to 

 one and the same meridian. 4. The general declension in level of the 

 Appalachian strata towards the N. W., or away from the quarter of 

 greatest local disturbance, is considered important by the authors in its 

 bearing upon the subject of the elevation of broad continental tracts. 

 The authors next proceed to notice memoirs, describing what they 

 consider similar phenomena in Europe. 



Theory of flexure and elevation of Strata. — From the consideration of 

 the preceding general facts the authors have arrived at a theory which 

 they conceive applicable to the bending and elevation of Strata gene- 

 rally. They state that the oblique form of all normal anticlinal and 

 synclinal flexures "indicates that the force producing the dips was com- 

 pounded of a wave-like oscillation and a tangential pressure;" — a 

 purely vertical force exerted simultaneously or successively along pa- 

 rallel 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 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 gase- 

 ous vapours ; the tension would at length be relieved by many parallel 

 fissures formed in succession, through which much elastic vapour 

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



4 D 



