SOME AUSTRALIAN EARTHQUAKES. 419 



South Australian earthquakes appear to illustrate very well the 

 general laws enunciated by De Montessus de Ballore, who may be 

 regarded as one of the best authorities on seismic geography — viz., 

 that : — " Most earthquakes occur where the variations of topographic 

 relief are greatest. (1) The most unstable regions are the mo^I; 

 pronounced general slopes, the short and steep flank of a chain being 

 the most unstable. (2) The unstable regions are associated with the 

 gi-eat lines of coiTugation of the terrestrial crust, either emerged or 

 submerged." 



Our South Australian recorded earthquakes seem to occur princi- 

 pally in the hilly and mountainous country east of Spencer's and St. 

 Vincent's Gulf, extending northward to the Lake Torrens district. 



In the geological map. on which they have been marked this 

 country is coloured slaty grey, and the rocks are principally clay 

 slates, calcareous clay slates, shales, sandstones, quartzites, grits, 

 conglomerates, limestones, dolomites, and kindred rocks, with granitic 

 and other igneous dykes, lodes, and mineral veins. They are metalli- 

 ferous rocks of the Cambrian, and perhaps Lower Silurian, period. 



Lying uncomformably on them, and forming the bulk of the sur- 

 rounding country'', are the following formations, which are marked pale 

 green on the map : — 



Blown sand of the coast and interior, sand, clay, loam, gravel, 

 marl, g-j^sum, mud, salt, travertine, and shell-limestone, 

 calcareous, and peaty deposits of springs and swamps, 

 sandstone, limestone, conglomerate, gravel, and bouklei- 

 drifts, and kindred deposits. 



Alluvial deposits, auriferous cement, and "deep leads" of the 

 goldfields. 



Lig-nite deposits. Limestone, clay, sand, calcareous sandstone, 

 limestone conglomerate, and breccia of the coast. 



These are formations of Post Tertiary, Pleistocene, and Pliocene ages, 

 for the most part sedimentaiy deposits laid down on the ocean floor. 



A large proportion of our earthquakes have been felt off the 

 borders of these unconformable rocks. The character of the isoseismal 

 lines of the 1902 earthquake in the whole of the districts east of St. 

 Vincent's and Spencer's Gulf marks this region as one more delicately 

 sensitive to seismic vibrations than other parts of our State. (Fide 

 Map IV.) The question arises : What is the probable cause of our 

 earthquakes? Our tremors seem to resemble in kind, though not in 

 degree, quakes resulting from the dislocation of great masses of the 

 earth's crust, like that of Bengal and Assam in June, 1897, when re- 

 mai-kable faulting took place, relative changes in the heights of hills 

 as great as 24 ft., and changes of 12 ft. in their Jiorizontal distances 

 having been observed. 



Investigations of the phenomena of this earthquake by the India 

 Geological Survey Department showed: — 



(1) " The absence of any strict epicentre, shocks of Number X 

 intensity (Rossi Fore! scale), being felt over about 6,000 

 square miles. 



