GENEEAL STRATIGEAPHY. 4-1 



regarding crust-movements. The Silliet trap too appears just in the 

 proper position, between the area of erosion and of deposition, on the 

 axis of oscillation or of weakness ; only, to be completely illustrative, 

 it should be intrusive into the younger deposits. It may, however, retain 

 its position in this relation, if we take a wider view of the case : it is 

 scarcely to be supposed that these liaes of disturbance were first struck 

 out at the recent period assignable to the formation of the Burrail range, 

 so that the trap -flows may belong to some much older period of activity 

 in the same system of depression and contortion. 



It is equally plain that the facts described afford a strong prima 

 facie example agaiast any rigid interpretation of M. de Beaumont's 

 theory upon the rectilinear character of cotemporaneous features of dis- 

 turbance : here we find one and the same great axis of flexure, some 

 hundreds of nules ra length, bending round at half a right angle. But 

 this objection in its turn may be qualified by the remark that the bend 

 occurs where the magnitude of the mountain range begins to decrease. 



The fact just noticed indicates another coincidence in illustration 

 of the theory that connects disturbances of strata and mountain forma- 

 tion with the accumulation of deposits. There is no doubt but that the 

 extension of the Burrail ridge westwards has been a good deal cm-tailed 

 by denudation; but it seems more than probable that this extinction 

 of the range is causally connected with the decrease of disturbance 

 that takes place in the same direction ; and that this again has a similar 

 relation to a decreased thickness of deposits. This decrease westwards 

 is not very clearly seen in the cretaceous strata. The coal-bearing sand- 

 stone is the only cretaceous rock in the western sections, and it is very 

 much thicker than the same band in the Cherra sections ; but then it is 

 not now known whether this rock in that region may not represent a 

 great part of the whole series at Cherra ; nor can it be known that the 

 marine beds of Cherra do not occur in the southern underground extension 

 of the formation in the Garo region. Again, the characteristic rocks of 



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