THE UNDERGROUND FORM OF FLOOR OF OANGETTO TROUGH. 75 



or about TOO miles south of the main boundary, but would probably 

 be more nearly under that station. Tt will, consequent! v, be 

 convenient to assume a width of 100 miles and a maximum depth 

 of 15,000 feet for the trough in this region, and the deflections 

 which should result from the effect of such a trough, combined 

 with the effect of the Imaginary Range, are shown in the table. 



So far as the stations from the north down to Chanduria are 

 concerned the agreement is very close, and the correctness of the 

 method of calculation is confirmed by Major Crosthwait's calcu- 

 lations which give residuals, after allowing for the effect of visible 

 topography and its compensation, of — 7" at Siliguri. + fi" at Jalpai- 

 guri, and + 10" at Chanduria, as compared with deflections of — 5" 

 -f 3" and -j- 8" allowed for the effect of the assumed trough in ob- 

 taining the figures of table 22. Southwards of Chanduria there is 

 a considerable discrepancy at Charaldanga, but here it must be 

 remembered that the width assumed for the trough was only 100 

 miles ; if the width is taken at 120 miles, the southerly deflection 

 at Charaldanga would be increased to about 5", provided that the 

 effect of the alluvium to the south was small, and this is not an 

 improbable supposition. 



The large southerly deflection at Madhupur. which is continued, 

 though in lesser amount at Calcutta, may well be due to another 

 cause than the effect of the alluvium. These stations lie in a 

 region where the geological structure, confirmed, as will be seen 

 further on, by the gravity observations, indicates that the depth 

 of the alluvium is probably small, and that we are outside the 

 limit of the Gangetic trough proper. At Calcutta a boring, which 

 reached a depth of 481 feet, met with deposits indicating the prox- 

 imity of a rock area, and it is probable that, over the tract separ- 

 ating the Peninsula from the Assam Hills, the depth of the allu- 

 vium is to be measured by hundreds, rather than thousands, of 

 feet, so that it can have little effect, either on the deflection of 

 the plumb-line or the force of gravity. We must, in fact, regard 

 this area as belonging, so far as deep-seated structure is concerned, 

 to the Peninsular area, and not to the Gangetic trough. 



At the stations north of Charaldanga the deflections are suffi- 

 ciently accounted for on the supposition of a trough shallowing 

 from a depth of about 15,000 feet near the hills to a shallow depth 

 of alluvium at about a hundred and twenty miles to the south of 

 them, and the conclusion may be drawn that the southern boundary 



[ 223^] 



