420 PROCEEDINGS OF SECTION E. 



(2) " The-great extent of the countiy affected, the shocks being- 

 distributed over an area of 1,750,000 square miles. 



(3) "A notable number of after shocks, the great quake being 

 - followed by repeated shocks of less intensity than the 

 ■. "V. primary one." 



Having due regard to the proportion of the force producing that 

 powerful disturbance (the greatest then recorded), to the foi'ce pro- 

 ducing even the inost widespread of South Australian tremors, one 

 cannot fail to note, in the latter as in the former, these three main 

 features ; and the geological confonnation of the countiy affected 

 supports this view — younger beds lying tmconformably against the 

 mountain ranges, as previously mentioned. 



In South Australia no dislocations visible to the eye have been 

 found accompanying earthquakes ; but the line of fault may lie in the 

 direction of Lake Tonens, Spencer's Gulf, and St. Vincent's Gulf. The. 

 raised beaches of our coast are evidence of former small and sudden 

 uplifts ; and it is quite conceivable that such movement is going on 

 to-day. 



The great Calabrian quake of 1783, w'hich shook all Sicily, and 

 which has found a parallel in the terrible catastrophe that so I'ecently 

 again devastated those unhappy regions, was plainly a tectonic or dis- 

 location disturbance. 



Earthquakes clearly attributable to eruptions of Mount Etna, 

 though violent near that volcano, have seldom been strongly felt across 

 the straits in Calabria. 



The geography and geoiog}^ of the district, as w'ell as the seismic 

 records, show that this is one of the regions illustrating De IMontessus 

 de Ballore's general laws previously quoted. 



Professor George Dai-win's thesis (Proceedings of the Royal 

 Society, June, 1881) gives some idea of the stresses caused by the 

 transfers of load. The age-long denudation of the Mount Lofty and 

 similar ranges, with the consequent deposit in our Gulf of the reiinoved 

 material, producing cumulative alterations of pressure and gravita- 

 tion, may cause stresses and stress-differences sufficient for sudden 

 down-thrust of the area of sedimentation and uplift of the denuded 

 regions (about a local line of pivoting?). 



Professor Gregory, writing to Sir Charles Todd, had little doubt 

 that the shock of 1902, concerning which Sir Charles wrote a paper 

 for your Association, was due to foundering under either St. Vincent's 

 or Spencer's Gulf like the 1897 earthquake. The epicentre of the 

 former was indeed, by all appearances, in St. Vincent's Gulf near the 

 foot of Yorke's Peninsula, and that of the latter, the 1897 one, was 

 off our south-east coast, in the vicinity of Kingston and Robe, (^vide 

 isoseismal Maps IIL and IV.). 



Similar causes have been, and still are, operating on both sides 

 of Bass Strait, and Professor David considered that " further cracking- 

 of the earth's crust might have caused the bed of the gulf to still 

 further fall at the point where it meets the Mount Lofty Ranges, or it 

 is possible that that point of the range mav have been squeezed up."' 

 (The "Register," 23rd September, 1902.) 



