HISTORICAL SUMMARY. 



357 



plateau, they certainly seem to bear some likeness to each other from a 

 geological, if not from a physical and spectacular point of view. Both 

 are composed, speaking generally, of rocks older than those of the 

 plateau beyond, and in both cases the main rivers, gathering on the 

 uplands, break across the strike of the rocks through profound gorges. 

 In each case the zone of older rocks is bounded by a great fault, 

 or series of faults, forming the inner edge of the ' foredeep' that 

 separates them from the ' foreland ' of the continent beyond. In 

 front of this again we have a zone of Tertiary strata, thrown into 

 folds and greatly dislocated by faults, in the one case occupied by 

 the Tertiary series of the Irrawaddy valley, and in the other by 

 the Siwalik strata of the sub-Himalaya and by the ' Duns,' those 

 wide valleys which separate the sub-Himalaya from the Siwalik 

 ranges. These ranges are represented in the Burmese area by the 

 Arakan Yoma, prolonged to the north into the parallel folds of the 

 Chin-Lushai Hills and the Patkoi range of Assam, and here the order of 

 magnitude is reversed, for in Burma these ranges attain to a far 

 greater altitude than in Northern India. Finally, we have in front of 

 these, on the one hand the broad Indo-Gangetic valley, and on the 

 other the Bay of Bengal and the swamps of Sylhet and Cachar, 

 the former completely, and the latter partially, filled with recent 

 alluvial deposits. 



The results of the compressive forces that affected the Shan 



_ i . .. , . plateau may be considered under two distinct 

 Tertiary diastrophism. f J 



heads : (1), the production of more or less reg- 

 ular folds, accompanied by overthrust or reversed faults parallel 

 to the strike of the folds, and (ii), vertical faults due to the sagging 

 down of the underlying Archaean floor under the influence of 

 gravity. These latter faults bear no relation to the direction of the 

 strike of the rocks, but follow straight or slightly curved lines 

 for considerable distances and are often at right angles to each 

 other, while they are frequently visible at the surface as long 

 lines of vertical scarp. 



The first type of dislocation seems to have preceded the second' 

 for the folds into which the rocks are thrown, especially among the 

 Eastern Ranges, are broken up by faults which cut directly across 

 the undulations of the strata ; but it also appears to have persisted 

 to a epiite recent period, geologically speaking, though to a very feeble 

 degree in later times, for the silts of the late Tertiary coal-basins 

 are distinctly tilted in places (Plate 1G). 



