The enormous dimensions of the parent focus will be obvious from the phenomena that have been described above. Mr. Oldham has traced the probable form of the epicentre. It may in reality be neither so simple nor so symmetrical as is represented in Fig. 75, but there are good reasons for thinking that it does not differ sensibly either in size or form from that laid down. The part of the thrust-plane over which movement took place must therefore have been about 200 miles long, not less than 50 miles wide, and between 6000 and 7000 square miles in area. With regard to its depth, we have no decisive knowledge. It may have been about five miles or less; it can hardly have been much greater. It is a strain on the imagination to try and picture the displacement of so huge a mass. We may think, if we will, of a slice of rock three or four miles in thickness and large enough to reach from Dover to Exeter in one direction and from London to Brighton in the other; not slipping intermittently in different places, but giving way almost instantaneously throughout its whole extent; crushing all before it, both solid rock and earthy ground alike; and, whether by the sudden spring of the entire mass or by the jar of its hurtling fragments, shattering the strongest work of human hands as easily as the frailest. Such a thrust might well be sensible over half a continent, and give rise to undulations which, unseen and unfelt, might wend their way around the globe. REFERENCES. 1. AGAMENNONE, G.--"Notizie sui terremoti osservati in Italia durante l'anno 1897 (Terremoto dell' India poco dopo il mezzogiorno del 12 giugno)." _Ital. Sismol. Soc. Boll._, vol. iii., pte. ii., 1897, pp. 249-293. 2. ---- "Il terremoto dell' India del 12 giugno 1897." _Ibid._, vol. iv., 1898, pp. 33-40. 3. ---- "Eco in Europa del terremoto indiano del 12 giugno 1897." _Ibid._, vol. iv., 1898, pp. 41-67. (See also the same volume, pp. 167-172.) 4. BARATTA, M.--"Il grande terremoto indiano del 12 giugno 1897." _Ital. Soc. Geogr. Boll._, vol. x., 1897, fasc. viii. 5. CANCANI, A.--"I pendoli orizzontali del R. Osservatorio geodinamico di Rocca di Papa, ed il terremoto indiano del 12 giugno 1897." _Ital. Sismol. Soc. Boll._, vol. iii., 1897, pp. 235-240. 6. HEATH, T.--"Note on the Calcutta Earthquake (June 12th, 1897) as recorded by the bifilar pendulum at the Edinburgh Royal Observatory." _Edinb. Roy. Soc. Proc._, 1897, pp. 481-488. 7. OLDHAM, R.D.--"Report on the Great Earthquake of 12th June 1897." _Mems. Geol. Surv. of India_, vol. xxix., 1899, pp. i.-xxx., 1-379, with 44 plates and 3 maps. 8. ---- "List of After-shocks of the Great Earthquake of 12th June 1897." _Ibid._, vol. xxx., pt. i., 1900, pp. 1-102. 9. ---- "On Tidal Periodicity in the Earthquakes of Assam." _Journ. Asiat. Soc._, vol. lxxi., 1902, pp. 139-153. FOOTNOTES: