261 



and accords with the succession of events that is assumed to 

 have taken place on the western side of the ranges. The 

 western slopes of the dissected plateau show successive steps 

 in which longitudinal segments have slipped down — in some 

 ■cases below sea-level— forming the foothills of the ranges and 

 the sunken areas of Gulf St. Vincent and Spencer Gulf. The 

 Mount Mary occurrences of these beds give evidence of similar 

 earth movements having taken place on the eastern side of 

 the ranges as on the western. A very distinct fault-scarp 

 faces the Murray Plains, with a north and south strike, 

 making a line with Eudunda. Point Pass, and Robertstown. 

 'To the eastward of this fault-scarp the ground slopes away 

 to the River Murray. Point Pass railway station, situated 

 at the foot of the eastern scarp, is 1,249 feet above sea-level; 

 "the oyster-bed, near Mount Mary, is 340 feet above sea-level ; 

 and the corresponding' bed at Morgan is 40 feet above sea- 

 level. Bv the movement of elevation an old sea-bed was 

 raised several hundred feet above sea-level, and this upward 

 movement was followed by longitudinal fractures, in two or 

 more steps, by which the faulted segments left the old marine 

 sediments at different levels. 



These differential movements of the earth's crust supply 

 a certain order of succession by which we may infer their 

 geological age. Thus the older marine beds of Cainozoic age 

 have been more displaced and occur through a greater range 

 of altitude than the newer marine beds of the series. These 

 facts suggest that the earth movements had been in operation 

 before the newer members of the series were laid down and 

 were continued subsequently to the latter's deposition. The 

 movements were evidently complicated, and involved alterna- 

 tions of elevation and subsidence. 



My attention was called to the Ostrea bed by my nephew, 

 Mr. E. E. Howchin, who had received his information from 

 Mr. A. Tonkin, whose duties as an enginedriver on the 

 Morgan railway took him daily through the cuttings where 

 the shells are exposed. 



