313 



near Trevilla, the residence of Mr. H. H. Blackham, One Tree 

 Hill ; at Mylor, on the Onkaparinga, and other places in the 

 Mount Lofty Ranges. 



It is not an easy matter to give a satisfactory explanation 

 of the structure in this case. While the Lower Cambrian 

 beds have been subjected to much distortion from tangential 

 pressure and thrusts, there is no definite evidence of such 

 having operated on the beds in question in a way that might 

 cause a dynamic deformation. It is more likely to have 

 arisen from some form of enterolithic action in which 

 infiltration and substitution have been the chief agents. 

 Secondary silicification probably took place when the black 

 carbonaceous shale became altered to chert and the 

 interpenetrating 'silica formed the reticulating veining that 

 has united the cherty fragments into a form of brecciated 

 rock. 



THE BRIGHTON LIMESTONE HORIZON. 



This is the most important limestone in the Lower 

 Cambrian Division, and. is remarkably persistent along a line 

 of strike that occurs, at intervals, over several hundreds of 

 miles, in a north and south direction, and is often repeated, 

 transversely, by a succession of foldings. The following 

 localities illustrate its occurrences : — 



Brighton. 



The limestone is typically developed in the neighbourhood 

 of Brighton, about 10 miles to the southward of Adelaide. 

 The main limestone is underlain by a thick series of banded 

 and siliceous limestones, and is overlain by the Purple Slates 

 Series, which form the lowest beds of the Upper Cambrian 

 Division. The limestone is in three fairly distinct layers of 

 about equal thickness, which, unitedly, give a section of about 

 50 feet. The lowest member is of a dark blue colour and 

 contains about 40 per cent, of silica. The middle bed is of 

 a dull pinkish colour, often oolitic in the grain, and carries 

 about 86 per cent, of calcium carbonate, the balance consisting 

 of silica with a small percentage of ferric oxide. The top 

 bed is generally of a buff colour, pinkish in patches, and is 

 dolomitic. 



The structure of this limestone, especially in the upper 

 portions of the middle bed, is subject to much variation^. Ati 

 Brighton, the middle bed, or ''pink limestone," is commonly 

 of a semi-oolitic structure, that can only be recognized by 

 the use of a hand lens. Under the microscope, in thin 

 sections, the groundmass is seen to be partly microcrystalline 

 and partly granular, and is thickly studded with more or less 



