388 Prof. W. G. Woolnough — Laterite in W. Australia. 



that the residual material is suitable for tlie manufacture of fire- 

 bricks. These, and brick arches for locomotive fire-boxes, are made 

 at Smith's Mill (15 miles E.N. E.) and at Clackliue (50 miles E.N.E.). 

 That the leached material is in situ is very clearlj^ demonstrated by 

 the preservation of minute aplite, pegmatite, and quartz Teiiis 

 through it (as at Hoddy's Well), and by the slight differences in 

 colour and consistency of the kaolin caused by the presence of basic 

 dykes through the granites. Some of these structures are only 

 a fraction of an inch in thickness; yet, though they have been 

 cracked into short sections, they preserve their continuity for 

 considerable distances. This is clear pz'oof of the residual character 

 ■of the pipeclay foundation of the laterite, and indicates that the 

 volume of the pipeclay is very little less than that of the rock from 

 which it has been derived. 



The laterite capping, as above noted, is usually from 3 to 6 feet thick. 

 In many instances where it has not been stripped off (for railway 

 ballast or road-making) the apparent thickness is greater because of 

 the collapse of the capping through erosion of the pipeclay substratum 

 at the edge of the outcrop. The laterite extends as larger and 

 smaller continuous cappings over the plateau areas which have not 

 yet come under the action of the dissecting streams. These cappings 

 are thus the residual portions of a once continuous sheet which 

 mantled the entire peneplain surface. 



As we pass inland, beyond the area which is in process of active 

 dissection by coastal streams, into the broad extent of undulating 

 country for tiling the wheat belt of Western Australia, the distribution 

 of crystalline rock and laterite becomes different from that near the 

 western scarp of the plateau. The characteristic elements of the 

 land surface are broad, exceedingly mature, meridional valleys 

 alternating with low ridges. The valleys are heavily aggraded, and 

 the slopes are well mantled with soil, though extensive outcrops of 

 granites and greenstones are also met with at intervals. The ridges 

 are partlj'' of the same character as the slopes, that is, soil-covered, 

 but are rather more than half composed either of " Sand Plain " or of 

 large granite outcrops. The latter (the granites) form immense flat 

 •domes, sometimes several miles in circumference, and have played 

 a very important part in the exploration and prospecting of the 

 country. On their surfaces are found the "rock-holes" and 

 " gnamma-holes " whence the earlier travellers obtained their water 

 supplies. The "Sand Plains" are extensive areas of light, friable 

 sandy soil, beneath which, at all events in many instances, occurs 

 a bed of sandy lateritic material. The perfect pisolitic structure of 

 the escarpment laterites is almost completely wanting in those of the 

 sand plain, thoa<ih well-defined concretionary structure is plainly 

 discernible. Solid masses of coucretiouary laterite forming extensive 

 mappings of the higher residuals are not encountered through the 

 wheat belt. 



Further east, again, the physiography alters once more. The 

 meridional valleys are no longer detinite stream channels, but have 

 degenerated into strings of salt lakes, whose floors, consisting for the 

 most part of comparatively insignificant thicknesses of detrital 



