164 president's address. — section c. " 



slates. A strike fault has thrown down the overlying slates and given 

 them a position lower than the quartzite, which should underlie them. 

 Usually, the quartzite, at the point of junction, makes a sharp fold, 

 following the downthrow of the slates. Again, the fault-plane, which 

 was probably, in the first instance, a normal fault, has become a 

 reversed fault by overthrusting towards the sunken area, as in the 

 Stonyfell quarries ; or an underfold, as in one of the Mitcham quarries. 



A still more striking illustration of a downthrow to the Gulf is seen 

 at the Sellick's Hill beach, 30 miles south of Adelaide. Here the older 

 cainozoic marine beds, which elsewhere are practically horizontal, 

 have been thrown down to the Gulf, at a very high angle, reaching the 

 vertical, and even overturned in the direction of the downthrow — the 

 imderlying cambrian slates have also participated in the same 

 movement.* 



Further confirmatory evidence of the instability of this section of 

 the earth's crust can be gathered from the earthquake records. The 

 meridional line of Gulf St. Vincent, with its transverse lines corres- 

 ])onding to the outlines of the chief block segments, may be regarded 

 as the most seismically unstable area in Australia. Slight tremors 

 are frequent, and within the period of European settlement two rather 

 severe earthquakes have had their epicentra ofE the shores of South 

 Australia ; one, in 1902, within the limits of the Gulf, and the other, in 

 1897, in close proximity to it.f Such seismic activity not only 

 shows that this longitudinal strip of country is a zone of crustal 

 instability, but suggests that the subsidence of the troiigh valley is 

 still in progress. 



We are unable to include in our present review Yorke Peninsula 

 and Spencer Gulf, as the data available for our purpose with regard to 

 these districts are very limited. However, it is worth noting that, 

 south of Ardrossan, the cambrian rocks, which are almost horizontal 

 away from the seaboard, are thrown sharply down as they near the 

 coast and pass below sea-level, which is suggestive of similar tectonic 

 movements having operated on the western side of Gulf St. Vincent, 

 as on its eastern. No bores have tested the ground around the shores 

 of Spencer Gulf, and the entrance to the latter gulf is largely bridged 

 by islands, composed of older rocks, that fail to give the same kind of 

 evidence for subsidence as occurs in Gulf St. Vincent. On the other 

 hand, the two basins have much in common, and we may reasonably 

 infer that their physiographical evolution has been along coincident 

 lines. 



One remarkable feature of the local geology is the apparent absence 

 0? the lower marine beds, at depth, from the southern end of Gulf 



* " Ddscription of a Disturbed Area of Cainozoic Rocks in South Australia with Remarka on 

 iii Gaological Significance." Trans. Roy. Soc. S. Aus., Vol. XXXV, pp. 47-69. 

 t Howchin's " Geography of South Australia," pp. 135-141. 



