81 



which they are found, but has thrown doubt on the assumption 

 that the azoic features of the Mount Lofty beds in general ar& 

 due to a question of age. 



The point that awaits determination now is — Are the funda- 

 mental rocks of the Mount Lofty Ranges comprehended in one 

 great Cambrian system, or are these rocks divisable into a newer 

 and an older series, a Cambrian and a Pre-Cambrian formation 1 

 If the latter, then we must find some line of unconformability by 

 fault or otherwise. 



Professor Tate has suggested the probability of a great fault 

 existing in these old rocks somewhere in the vicinity of Adelaide. 

 I believe he has been led to an inference of that kind partly from 

 the great depth of the bed rock in the Croydon bore, amounting 

 to 2,000 feet. The same rocks in the Adelaide bore were proved 

 at a depth of 360 feet, which indicates a gradient of 1,640 feet in 

 a distance of about three and a-half miles. It is not impossible 

 that erosive agencies may be responsible for so great a difference 

 of level, and evidence is not wanting to prove a similar rapid 

 descent of the old rocks below the plains on the eastern side of 

 the ranges,* yet the phenomenon at Croydon is a remarkable one, 

 and may give the clue of a great crust movement, the confirma- 

 tion of which must be looked for in our hill country. A great 

 downthrow of the beds is capable of preserving outliers of a 

 newer formation, and in this way it is not impossible that Cam- 

 brian beds may, by faulting, be thrown against a face of Pre- 

 Cambrian rocks. This is one of the problems which the present 

 discoveries have raised, and which can only be settled by careful 

 and extended observations in the field. 



At present no such line of fault is known to exist, and there 

 are some stratigraphical features which seem to indicate that the 

 fossiliferous beds described in this paper are interstratified and 

 conformable with the great geological system of the hills. We 

 have, for example, in the Normanville and Sellick's Hill district 

 a continuous outcrop of Lower Cambrian rocks for a distance of 

 over twenty miles. The general strike and dip of these beds are 

 homologous with that which characterise the hill country in 

 general, and the lithology of the beds bears a close resemblance 

 to the shales, limestones and quartzites of the central and 

 Western parts of the ranges. 



As bearing upon the present discoveries, special interest cen- 

 tres in a wide belt of limestones that takes in Brighton, Field 

 River, Reynella, and Noarlunga. These beds vary in composition 



* A bore put down at the Pine Hut Creek, on the Murray Flats, within 

 a quarter of a mile of the foot of the hills had to penetrate a depth of 530 

 feet before reaching the bed rock. 



