CAUSE OF EARTHQUAKE. 335 



With regard to the smaller subsidiary focus for the isolated Dehra- 

 Mussoorie area, the fact that the latter is delineated by only one 

 isoseismal, which has only been crossed by my colleagues at one point of 

 its southern limb, and at a time when its isolation from the Kangra- 

 Kulu epicentral tract was not suspected, renders impossible the applica- 

 tion of Dutton's method of estimating the depth, inasmuch as there are 

 no data for approximately determining the points of most rapid 

 decline of intensity. On general grounds one would expect the depth 

 of the centrum to be consideiably less than at the Kulu end of the 

 northern area. 



IV.— CAUSE OF THE EARTHQUAKE. 



In touching on an obscure question of this kind in a publication that 



. , is more properly concerned with observed facts, it 



problem ; geological, seems to the writer that all he can do without 



structural and oroge- De mg too speculative is to show now the whole sur- 



nic features. . 



roundmgs of the case, geological, structural and 

 orographical, bear on some standard theory having an a priori prob- 

 ability. It will therefore be my endeavour in the following paragraphs to 

 briefly review those surroundings, and attempt to bring out any leading 

 features they may possess, and to show in which way they point. The 

 indications will be seen to be largely circumstantial but at the same 

 time suggestive. 



The geology of the area was roughly but comprehensively worked out 

 Geology of the by Mr. Medlicott as long ago as the year 1864. 1 No 

 area - fresh detailed geological survey of it has since then 



been made, except in isolated portions near Dehra Dun and further 

 south-east along the foot of the Himalaya ; although enough has been 

 done to illustrate the type of geological conditions that prevail along 

 that line of country, 2 whilst a few additional notes collected during 

 my present tour have been incorporated here and there in Part I. From 

 the above we can easily construct a bird's-eye view of the geology 



1 Mem. Geol. Surv. of India, Vol. III. 



2 For summary, see Manual Geol. of India, stratigraphical and structural, 2nd 

 Edition, Chapter XVIII. 



