253 



included quartz and other joebljles exhibit the same mineral 

 of primary origin in situ. 



At the base of the conglomerate, which is about 150 feet 

 in thickness, is a layer of finer material a few feet thick, 

 making an unconformable junction with the granitic and 

 highly-altered schists and quartzites on which it rests. 



The evidence of strain and shear, so generally present in 

 the Mount Lofty Ranges, is strongly developed in the lower 

 parts of the conglomerate. The line of junction with the 

 older Pre-Cambrian beds appears to have shown itself a plane 

 of weakness, and consequently of yielding along the line of 

 least resistance. The basal portions of the beds have been 

 greatly altered by shear, flattening out the particles, and 

 drawing them out in the direction of the movement. The 

 effect has been to convert the lower parts of the bed into a 

 flattened, schistose structure. A similar effect has been pro- 

 duced on the included pebbles within the zone of shearing, 

 flattening and drawing them out into long blade-like lenticles ; 

 whilst some of the quartzite pebbles have been converted into 

 quartz-rock, making pseudo-quartz veins along the planes of 

 bedding. The effect is most striking. In the upper parts of 

 the conglomerate the included pebbles have suffered little or 

 no distortion, but as they gradually approach the shear 

 plane for some yards the deformation becomes increasingly 

 evident. At one spot, near the bottom of the bed, differential 

 movement could be detected in a line of fracture which passed 

 through three adjacent pebbles, with the effect that the upper 

 portions were carried forward 2 inches beyond those portions 

 of the pebbles which were situated below the line of fracture. 

 No intrusive veins of quartz were observed passing up 

 from the older beds into the Cambrian grits, although it is 

 probable that the shearing took place at great depth, and was 

 associated with some measure of hydro-thermic action, indi- 

 cated by the development of quartz along the bedding planes, 

 and which no doubt contributed to the plasticity of the 

 pebbles under pressure and movement. 



The conglomerate bed has been greatly fractured, ex- 

 hibiting vertical smooth joints, the joint planes passing equally 

 through matrix and pelDbles, showing as clean and smooth 

 faces as though cut by a knife. 



The lower, or Pre-Cambrian, beds in this section con- 

 sist cJiiefly of aplite in coarse crystals of quartz and felspar, 

 sometimes passing into pegmatite or granite. The beds are 

 much broken by quartz veins, which, together with the granite 

 intrusions, have penetrated and greatly altered the sedimen- 

 tarv beds of this older series. 'The external appearance of 

 these beds is very deceptive, for the molecular reconstruction 



