266 HOBBS— THE EVOLUTION AND THE [April 24. 



We must, therefore, regard the cause of the earthquake of June 11, 1895, 

 as a movement of the Nimpt complex of orographic blocks, which occurred 

 along the southern and eastern fracture margins/ 



The great Indian earthquake of 1897 was thoroughly examined 

 from the geological side with results which seem to have afforded 

 indication of the movement of the ground in individual blocks. 

 This, however, was not the theory adopted by R. D. Oldham, who 

 wrote the report upon the earthquake, apparently for no other rea- 

 son than that it seemed to require an expansion of the affected area. 

 In consequence, the unique hypothesis was oft'ered that the earth- 

 quake was due to a movement upon a thrust plane beneath the 

 affected region. The mental attitude of Dr. Oldham is brought out 

 in the following paragraphs from his report in modification of his 

 choice of theory :^ 



Though apparently the most probable this is not the only possible, 

 hypothesis. The surface features of the Assam range, described in the last 

 chapter, are compatible with, in some respects tliey suggest, the idea that these 

 hills are what the German geologists call Schollengebirge, that is, mountains 

 which have arisen from straight up and down thrusts, instead of from lateral 

 compression, like the Alps and Himalayas. // this be so, the faults by which 

 the fault scarps are formed would be normal faults^ and so far from there 

 having been any compression, the elevation of these hills would have been 

 accompanied by an extension of the surface. The state of strain, too, which 

 preceded the earthquake would have been one of tension and not" compression. 



The mechanism of the production of this form of mountain is not prop- 

 erly understood, and a condition of tensile strain in the crust of the earth 

 would be still more difficult to explain, but the fact of the existence of such 

 mountains and structures cannot be gainsaid, so the possibility of the state 

 of tensile strain they imply must be allowed. 



// such is the nature of the Assam range, and of the cause of this earth- 

 quake, there would be no thrust-plane underlying it, and the focus of the 

 earthquake would have to be regarded as a complex one. That is to say, 

 there would be no general focus, but a number of independent ones, along 

 each fault, and the magnitude of the earthquake experienced zvould be due 

 to the simultaneous occurrence of a number of earthquakes of various 

 degrees of severity. 



Whether we regard the focus as a thrust-plane, or as a nctivork of faults, 

 it practically covered an extensive area.'' The hypothesis of a thrust-plane 



* Zeitsch. f. Erdkunde s. Berlin, Vol. 31, 1896, pp. 1-21. 

 ■^R. D. Oldham, "Report on the Great Earthquake of 12th June, 1897," 

 Mem. Geol. Surv. India, Vol. 29, 1899, pp. 165-168. 

 ' The italics are mine. — W. H. H. 

 ' The italics are mine. — W. H. H. 



