44 MEDLICOTT, SHILLONG PLATEAU. 



the structure of this ridge has all the appearance of a normal or a 

 folded flexure, of which the axis-plane underlies to the south^ towards 

 the area of depression, and thus points to the direction of the lateral 

 pressure, from that area. 



It appears at once from the several sections described, that features 

 such as that just indicated are exceptional ; that most of the normal 

 flexures have their axis-planes sloping northwards. It wiU be recollected 

 that Mr. Rogers (whose terminology for the forms of plications I adopt) 

 regarded flexures as true undulations proceeding from the focus of dis- 

 turbance, arrested and fixed by lateral pressure. -By such ^ mode of 

 formation, the axis-plane must incline towards the origin of the wave ; 

 but Mr. Rogers' hy3)othesis of causation was scarcely maintainable. 

 Still it might be considered that in any natural process for the produc- 

 tion of unsymmetrical flexures, the axis-plane must, from first to last, have 

 that constant relation to the source of the pressure, — that the shorter and - 

 steeper side of the anticlinal curve should be on the side further' from 

 the compressing force. It is most unlikely that any such absolute 

 rule should exist, the conditions of action and of resistance being so 

 variable ; and it is only by the examination of the most simple cases 

 obtainable that we can arrive at rules for general guidance, by which 

 we may be able to detect exceptional cases where the form of a flexure 

 was determined initially by peculiarity of position. In discussing else- 

 where (Mem. Greol. Surv., India, Vol. Ill, Pt. 2) a phase of this question 

 as to the forms and causes of contortion in which the pressure is that 

 exercised by a mountain-mass upon the strata at its base, it was seen 

 that the axis-planes of the flexures do conform to the rule of imderlying 

 towards the source of pressure, although it did not appear (page 134) 

 that when the flexure became faulted, the upthrow was always on the 

 same side as required by Mr. Rogers' hjrpothesis. Allusion was made 

 (page 170) to what might be the case where the contorting force was 

 that derived from the sinking of the strata themselves. It would seem 



( 194 ) 



