GENERAL DESCRIPTION OF AREA AND ROCKS. 13 



ably from each other iu hardness, the rocks composing any one formation 

 are tolerably uniform in this respect. The gneiss is as a whole consider- 

 ably harder than the slates, and the latter than the Tertiaries ; but, 

 excepting some not very important bands of quartzite in the slates, 

 there is nowhere, on a sufficiently large scale to materially affect the 

 orography, any alternation of strata of widely different hardness in the 

 same formation. Hence, in as far as the orography has been influenced 

 by the geological succession of rocks, it has been mainly so by the 

 succession of formations or series, not of minor sub -divisions. It will 

 be seen from the map that the lower Tista valley has been excavated 

 through the slates, the river, south of its junction with the great Rangit, 

 having selected for its course the axis of the transverse anticlinal* 

 alluded to above (p. 11). It seems not improbable that the Tista 

 between the Rang-chu and the Rangit, and the last-named river below 

 Gok, also flow along anticlinal axes, but the country to the north has 

 not been examined. 



The gradation in hardness of the several formations as we ascend 

 from the plains has also had a prominent influence on the elevation of 

 the outer hills. If a view be obtained of these looking east or west, 

 or parallel to the direction of the range, say from Pankabari bangalo 

 across the Balasan to the hills between that stream and the Mechi, it 

 will generally be found that those composed of Tertiaries seldom rise 

 more than two thousand, and often not more than a few hundred 

 feet. From the junction of the older rocks with the newer, the hills 

 rise more quickly to the outer limit of the gneiss, from which they, 

 often spring rapidly to a total elevation of several thousand feet. 

 Where the Tertiaries are absent between the Jaldoka and Langti rivers, 

 and the gneiss comes closer than usual to the base of the hills, the latter, 

 rise at once to this altitude. 



( 13 ) 



