TRANSACTIONS OF THE SECTIONS. 41 



by the nature of the flexures, which are not symmetric, the strata being more inclined 

 on the N.W. than on the S.E. of each anticlinal, amounting at length to a complete 

 folding under and inversion, especially on the S.E. side of the chain, where the con- 

 tortions are so closely packed as to present a uniform dip to the S.E. These folds 

 gradually open out, the N.W. side or inverted portion of each flexure becomes verti- 

 cal, or dips abruptly to the N.W. ; proceeding further in this direction the dips gra- 

 dually lessen, the anticlinals and troughs becoming rounder and flatter, and the in- 

 tervals between the axes constantly increasing till they entirely subside at about 150 

 miles from the region of gneiss and intrusive rocks. The authors express their belief 

 that a similar obliquity of the anticlinal axes will be found to obtain in all great 

 mountain chains, the axis plane always dipping towards the region of chief disturbance. 

 The inverted flexures are regarded by the authors as exhibiting simply a higher de- 

 velopment of the same general conditions. The passage of inverted flexure into 

 faults is stated to occur frequently, and invariably along the N.W. side of the anti- 

 clinal or S.E. of the synclinal axes j these dislocations, like the axes, maintain a re- 

 markable parallelism. 3. The axes of the Appalachian chain are distributed in na- 

 tural groups, the members of each group agreeing approximately in length, curvature, 

 amount of flexure and distance apart. Nine principal groups are described, in five of 

 which the axes are straight, whilst the four which alternate with them are curved j 

 in two of the curved divisions the line of strike is convex to the N.W., in the other 

 two it is convex to the S.E. In every part of the chain, the axes, whether curved or 

 straight, maintain an approximate parallelism to those of their own division, and in 

 the minor 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 fifty degrees in a distance of ninety 

 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 great circle of the sphere. 

 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 

 phaenomena in Europe. 



- Theory of Flexure and Elevation of Strata. — From the consideration of the pre- 

 ceding general facts the authors have arrived at a theory which they conceive appli- 

 cable to the bending and elevation of strata generally. They state that the oblique 

 form of all normal anticlinal and synclinal flexures "indicates that the force produ- 

 cing the dips was compounded of a wave-like oscillation and a tangential pressure;" 

 — a purely vertical force exerted simultaneously or successively along parallel lines 

 could only produce a series of symmetrical flexures, whilst tangential pressure, unac- 

 companied by a vertical force, would result in an imperceptible bulging of the whole 

 region, or in irregular plications dependent on local irregularities in the amount of 

 resistance. The alternate upward and downward movements 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 direction beneath the earth's 

 crust." With this view all the phaenomena observed are in accordance ; but it would 

 be difficult to account for them by any gradual prolonged pressure exerted either 

 vertically or horizontally. The formation of grand yet simple flexures cannot be ex- 

 plained by a repetition of feeble tangential movements, which could not successively 

 accord in direction and amplitude, nor by merely vertical pressures, for these could 

 not shift in position through a series of parallel lines, nor if feeble and often repeated 

 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 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 of fracture, produce violent pulsations on the surface of the fluid below. 



