144 COIililESIPOITZDIEIsrOE. - 



RAPID ELEVATIONS: A SELF-CRITICISM. 



Sir, — In the Geol. Mag. for 1892 (p. 406), I published a note on 

 the possible effects from Eapid Elevation of submerged lands, in 

 reference to Prof. Prestwich's paper on the Bubble Drift of southern 

 England. In it I said: " Most violent JajDanese earthquakes exhaust 

 their potency in vibrations measured by inches or less." I am not 

 aware that any measurement of vibrations have, even now, given 

 any larger values. But after reading a paper by Prof. Koto on the 

 great Japanese earthquake of October, 1891, it does not seem correct 

 to say that their potency is exhausted in small vibrations. Prof. 

 Koto gives both descriptions and photographs of effects from this 

 prodigious convulsion, and these include the formation of a visible 

 fault-line more than 70 miles long, with a relative horizontal dis- 

 placement of its two sides along its direction, of from 3 to 6 or 

 even as much as 12 feet in length, and with a relative vertical 

 displacement producing a step along the fault, whose height some- 

 times reached 20 feet. 



Prof. Koto gave attention to the question whether this step was 

 due to a rise of the region on one side, or a fall of that on the 

 other. We might expect that the effects on watercourses would 

 have afforded a criterion; however, he judges that the evidence is 

 insufficient for a decision. He regards fall as more probable, but 

 mentions that, while in general the western side of the fault is 

 lower, along one portion it is higher, and concludes that here at any 

 rate must be an uplift, for which accordingly he conjectures an 

 explanation. His conclusion is natural and probable. Still it is 

 not quite impossible that the cause of the dislocation may here have 

 changed sides of the fault. If water be drawn off from a frozen 

 canal, the sinking ice breaks with a longitudinal crack; but we 

 should not be surprised to find the two sides of the crack irregular 

 in their relative levels. Prof. Prestwich's theory assumes rapid 

 elevations. The throw of this fault is thus not proved to be an 

 elevation. Also how rapid was it ? This most interesting question 

 does not seem to have occurred to the investigator. He describes 

 a great shock, followed by less intense repetitions so numerous that 

 100 were counted dui-ing the remainder of the day, and 300 in the 

 dav following. Their frequency diminished, yet so gradually that 

 after ^the lapse of a fortnight they still averaged more than one per 

 hour. Sui-ely these numerous disturbances had a share in the visible 

 eifect. What part was produced by them, what part by the great 

 convulsion ? How nearly was the change " instantaneous ? " The 

 slope of the fault-step is described as being the angle of repose 

 of the material ; but this would be the natural result of frequent 

 shocks, and affords no argument as to the change being a single 

 effect. Any evidence on this subject would be most interesting and 

 valuable. 



It appears, then, that this Japanese Earthquake does not afford 

 an undoubted instance of such a Kapid Elevation as Prof. Prestwich's 

 theory assumes. Nevertheless the argument in my note against the 

 credibility of the theory is weakened, for one of my main data is 

 now not unquestionable. E. Hill. 



The Recxoky: Cockfield, R.S.O. Suffolk. 



