CAUSE OF EARTHQUAKE. 339 



being important from the point of view of the cause of the earth- 

 quake : — 



(1) The two epicentres lie in the Sub-Himalayan belt, approxi- 



mately but not exactly along the line of the main bound- 

 ary fault. 



(2) They are located near where that belt widens and shows 



inbayings towards the older Himalayan mass. 

 ^3) The main epicentre accords with the larger inbaying and the 

 subsidiary epicentre with the smaller. 



(4) They are both regions of marked and general change of 



average level of the country and average steepness of the 

 valleys and ridges. 



(5) They are also regions of reversed faulting, where a packing 



of the strata and an overriding of the younger by the 

 older rock series is specially prominent. 



(6) They coincide with parts of those regions where there is 



irregularity in that packing, and where the regular mar- 

 ginal arc of the mountain, as expressed in the parallel 

 earth folds and faults, is interrupted. 



It is now only necessary to recognise and give full value to the 

 Isoptasy. principle of isostasy of the earth's crust, as is clearly 



demonstrated by all the later work on the fringing ranges of the 

 Himalaya, in order to see the one direction in which all the features 

 enumerated above point. With reference to that principle it must also 

 be remarked that it appears to be not without probability that such 

 an area during the packing process incident on lateral compression of 

 the crust might at certain places temporarily lose its condition of isos- 

 tasy as regards the subjacent more mobile layers, and become support- 

 ed for in part supported) after the manner of an arch, a condition 

 which sooner or later must become unstable and a grand fracture (not 

 a net- work of surface fractures), or even a simple sudden readjustment 

 of strain, take place. 



Whilst non-mountainous continental areas of no lateral compression 

 may be supposed to hold together only so long as shrinkage of the 



z2 



