472 PROCEEDINGS OF SECTION C. 



band of quartzite is in parts fractured, but still incoherently in contact, 

 yet in parts consists of false pebbles of identical material, rounded and 

 separated from one another. The major axes of these pebbles usually 

 point in the same direction. 



Another method of formation of cataclastic pebbles is illustrated 

 in the section along the tributary of the River Sturt, at the eastern 

 boundary of the conglomerate. Stretched bands of arenaceous rocks, 

 in parts retaining their continuity, in parts dragged out into a series 

 of lenticles, simulate a string of large pebbles. The eastern portions 

 of the conglomerate itself contain many of these false pebbles. Strings 

 of pebbles and large " floating " masses of quartzite can be seen in 

 the 10th mile cutting and the one containing the tunnel close to the 

 Metropolitan Brick Works. " Pebbles " occur in lines parallel to the 

 cleavage, and with their longer axes in the same direction, and are 

 composed of rock identical with that of the beds bordering on the 

 conglomerate. They consist mainly of fine-grained quartzite covered 

 by a brown mineral film with a chalcedonic lustre, and of decomposed 

 felspathic grit or arkose, probably pegmatitic, both of which are con- 

 spicuous in situ in the adjacent beds. 



Quartz veins have undergone like deformations by stress. 



Similar phenomena can be traced in zones through the heart of 

 the conglomerate in the Sturt Valley. The pebbles are mainly oblong- 

 ovate in shape, often having their faces grooved by " slickensides," 

 and frequently the matrix surrounding the pebbles shows in section 

 " lines of flow," this characteristic being repeated to a microscopic 

 scale. The pebbles have been sHoed along parallel planes oblique to 

 the cleavage of the matrix ; these sheared planes have, however, no 

 definite relationship to the planes of cleavage of the matrix. 



Whatever may have been the origin of the conglomerate, whether 

 original or secondary, or partly both, a careful consideration in the 

 field of the phenomena above described will, no doubt, convince members 

 of this association that cataclastic action has played its full part in the 

 formation of " pebbles " in the conglomerate. 



DESCRIPTION OF PLATE. 

 Fig. 1. Pinched out quartzite bands producing pebbles by small 

 faults ; railway cutting, Blackwood. 



Fig. 2. False pebbles, railway cutting, Blackwood. 



