THE UNDERGROUND FORM OF FLOOR OF GANGETIO TROUGH. 73 



edge of the hills and probably about 80 from the main bound- 

 ary. This group of stations indicates an upward slope of the 

 floor of the trough at about 200 feet per mile, or a little less, which 

 would correspond to a maximum depth of about 20,000 feet, and 

 so far the observations accord with what was found further west. 

 The southern part of the section, however, shows some remarkably 

 anomalous features ; instead of a decrease of southerly deflections 

 at, and beyond, the southern limit of the trough, we find the very 

 high southerly deflection of 17" at Bihar and at Teona of 15". 

 It might be possible to explain these deflections by the effect of 

 the alluvium alone, if we assumed that the upward slope of the 

 floor of the trough was not continuous to the southern margin, but 

 ended in an abrupt rise of some 1,500 to 2,000 feet, an explanation 

 which is not geologically impossible, for the boundary of the rock 

 area, between Teona and Bihar, lies on the continuation of the 

 Son Valley, the line of flexure marking the boundary between an 

 area of elevation, to the south, and of depression, to the north. 

 This structural feature is of great geological age, but it is not im- 

 possibly continued under the alluvium to beyond Bihar, and move- 

 ment may have taken place along it during the formation of the 

 Gangetic trough. 



Though not impossible, this explanation is decidedly improbable, 

 even if looked at from a geological point of view alone ; regarded 

 as an explanation of the high southerly deflections at Bihar and 

 Teona it might be sufficient, it might even suffice for that found 

 at Mahar, but it fails altogether when the southern stations are 

 considered. Hurilaong, for instance, gives a deflection of 15", 

 but if the 17" at Bihar were entirely due to the alluvium, 

 the deflection at Hurilaong should not exceed 4", at Bulbul 

 2" and at Chendwar and Mahwari less than a second ; even 

 at Mahar it would take some forcing of the hypothesis to get a 

 deflection of more than 10". It is evident, therefore, that 

 something besides the alluvium is at work, and that we are within 

 the range of influence of an excess of density, in the rock area to 

 the south of Bihar. 



The high southerly deflection at Bihar is, therefore, made up 

 of two parts, of which some 9" or 10" may be attributed to 

 the effect of the trough and the remainder to some other 

 cause, such as an underground excess of density to the south. At 

 the stations north of Bihar the effect of this last-named cause must 



L 221 ] 



