378 SILICIFICATION OF GRAVELS 



trated in the overlying rock. In some cases the traces 

 of cleavage and joint planes are to be seen in the 

 bleached material, which must then be regarded as a 

 product of weathering in situ of slates ; but most of the 

 clays appear to be alluvial deposits formed by the rivers 

 when they were at a relatively higher level than at 

 present. 



The ferruginous material is closely related to the lat- 

 erites that occur at lower levels in many parts of the 

 south-west, and which will be described on a later 

 page. 



Another very widespread variety of the gravels is due 

 to the deposition of silica in the matrix subsequently to 

 the formation of the gravel. All stages between a 

 rather incoherent conglomerate and an extremely hard 

 rock from which it is practically impossible to detach 

 the contained pebbles can readily be found in one and 

 the same patch of rock. The deposition of silica is most 

 advanced on the upper surface of, the mass, the lowest 

 part of which is often a loose gravel. By the diminution 

 in size of the pebbles and their gradual disappearance 

 as the outcrops are followed away from the mountains 

 the quartzitic gravels pass into the typical "surface 

 quartzite " so widely distributed throughout the western 

 and southern parts of the Colony (see Plate XXIV.). 



As a rule the surface quartzites have certain pecu- 

 liarities that enable one to recognise the smallest chip 

 without difficulty ; their fracture is smoother, more con- 

 choidal, and less splintery than that of the quartzites 

 of the older formations ; small quantities of argillaceous 

 matter, yellow or grey in colour, are present in the 



