53 HUGHES. — KAilANPtJRA COAL-FIELDS. 



generally belong to the highest formation developed in the field^ from 

 which it might be inferred at first sight that the whole of the under- 

 lying group of strata have been thrown to the extent of their known 

 exposed thickness. This inference would probably be accepted without 

 much question^ if we did not interpret properly the indications of gradual 

 overlap which the formations exhibit in each field. 



In the reports of the Ranigauj, the Jherria^ the Bokaro, and the 

 Karanpura areas, instances may be multiplied of the gradual thinning 

 out of the difierent groups. The Damuda series and the Lower Panchets, 

 as they extend westward from the Raniganj district, exhibit " a steady 

 diminution in vertical development, which can be shown not to be due to 

 denudation ; for, in no ease can the contact beds of any two series- be said 

 to be unconformable in the sense that implies upheaval and . degradation 

 of the lower rocks previously to the deposition of the newer. The un- 

 conformity, as has been insisted upon again and again in my memoirs 

 on the Damuda valley and by others also, is due ' to simple overlap 

 and to irregularity cf deposition,^ facts which are, to quote Mr. 

 Medlicott^s words, ' compatible with a close relationship among the 

 groups as a series.' 



If we push to its extreme length the conclusion that may be drawn 

 from this evidence of gradual thinning, it would of course make all 

 the southern boundaries of the Damuda valley coal basins natural 

 boundaries, not faults, and it would remove any speculation as to the 

 nature of the original extension of the coal-measures ; but unfortunately 

 we cannot ascertain completely the rapidity of decrease, and several 

 features of the rocks point to the operation of subsequent disturbing 

 forces modifying the original boundary. We find also that on the north 

 edge of the field, where the boundary is admittedly natural, the rocks 

 exhibit less disturbance than on the south. 



We naturally look to some cause for this contrast, and the one that 



most readily suggests itself is disturbance, such as always results in 

 ( 336 ) 



