OP THE SHAPE AND EXTENT OF THE SEISMIC AREA. 87 



structure of the country does not offer any special impediment to the 

 transmission of the shock, but it is on the west that the shock was trans- 

 mitted for the greatest distance. Westward from the Khasi hills the 

 crystalline rocks soon give place to the great alluvial plains of the 

 Ganges and Brahmapootra which, with those of the Barak, surround 

 the Khasi range on three sides and form a very large portion of the area 

 affected by the shock. These alluvial plains extend westward, far beyond 

 the furthest point at which the shock was felt ; but in the south-west 

 the shock was felt over a district in which the gneiss of the peninsula 

 forms the groundwork of the country, in whose hollows are lodged the 

 principal coal-fields of British India. On the gneiss, which, from its elas- 

 ticity, is well adapted for the tansmission of the earthquake-wave, the 

 furthest point at which the shock was felt was Hazaribagh, 454 miles 

 from the seismic vertical ; while Patna, to reach which the shock would 

 have to travel for two-thirds of its journey through the alluvial plains of 

 the Ganges, is 446 miles from the seismic vertical, from which it would 

 eeem that the shock would travel as far through alluvial plains such as 

 those of the Ganges as through the highly crystalline gneiss of the pen- 

 insula, the absence of joints in the former case compensating for defi- 

 cient elasticity ; we must therefore consider the country to the west of 

 the Khasi hills as rather suitable than otherwise for the transmission 

 of the earthquake- wave. 



Though, as has been shown, the nature of the country, whether as to 

 its structure or accessibility, would to a certain extent account for the 

 east and west elongation of the isoseismal, it is not in itself sufficient 

 and the facts noticed cannot be explained without calling in the action 

 of some other cause. If, as it would seem to be the case from an inspec- 

 tion of map No. 1, the seismic focus took the shape of a rent running 

 nearly east and west, the impulse communicated to the surrounding 

 rock would be greater in a direction north and south than in the ortho- 

 gonal direction of east and west, and the impulse would tend to propa- 

 gate itself further in the former than in the latter direction. On the 

 north we cannot hope to trace any great extension of the isoseismal, but 



( 87 ) 



