Prof. W. 0. Woolnough — Later ite in W. Australia. 387 



them the structure becomes complicated in marked contrast to its 

 extreme simplicity within the specified zone. The escarpment has 

 an extremely uniform altitude of about 900 feet above sea-leve], and 

 marks the boundary between the Coastal Plain on the west and the 

 "Darling E.ange" on the east. 



The Coastal Plain is very uniformly about 15 miles wide and 

 consists almost exclusively of sandy deposits of recent age. In the 

 neiglibourhood of Perth these have been proved by artesian bores to 

 extend to a depth of at least 2,000 feet below sea-level. The surface 

 of the Coastal Plain is undulating, but does not, as a rule, rise to more 

 than 150-200 feet above sea-level. 



The name " Darling liange " is really a misnomer for the highlands 

 to til e east of the scarp. These highlands form actually one of the most 

 ]ierfect peneplains in the world, and they will be referred to, therefore, 

 throughout this communication as the Darling Peneplain or Darling 

 Plateau. The surface of this unit is gently undulating for the most 

 part. Its average altitude increases gradually as we proceed east 

 and north, so that it is 1,046 feet at Merredin (145 miles E.N.E.), 

 1.40afeet at Coolgardie (320 miles E.N.E.), 1,606 feet at Laverton 

 (450 miles N.E.), 1,755 feet at Sandstone (330 miles N.N.E.), and 

 1,708 feet at Meekatharra (400 miles N.N.E.). The plateau is built 

 up almost exclusively of extremely ancient crystalline rocks, some of 

 which are of acid composition (granites and gneisses), others of which 

 are basic (quartz-dolerites, epidiorites, "greenstones," etc.). 



The escarpment is deeply trenched by streams which flow westwards 

 into the Indian Ocean. The larger streams like the Swan, Helena, 

 Murray, Brunswick, Collie, and Preston, have reached base level 

 and have begun to widen their valleys just within the edge of the 

 plateau. The smaller streams and all but very insignificant stretches 

 of even the larger ones are, however, strikingly juvenile throughout 

 their intra-plateau portions. The whole of the western part of the 

 plateau is therefore intensely dissected and roughened by deep 

 narrow gorges. This is an important point to remember in discussing 

 the origin of the laterite. The zone of intense dissection is not very 

 wide, and, from Chidlow's Well (20 miles E.JSr.E.), the levels on the 

 Eastern Goldfields Railway (to Coolgardie, Kalgoorlie, and Laverton) 

 indicate clearly the very slight relief of the area. Yery rarely indeed 

 do depressions fall more than about 200 feet below the average level 

 of the surrounding country. 



In the western zone of the plateau crystalline rocks in situ are met 

 with only in the valleys of the young rivers which have dissected the 

 surface. As soon as the peneplain level is reached the surface is 

 covered with a dense shield of laterite. As Simpson has pointed out, 

 the composition of the laterite varies sympathetically with that of the 

 underlying bed-rock ; where the latter is granitic the laterite is 

 aluminous, where it is basic the laterite is ferruginous. In every case 

 the laterite, which is usually solid for a thickness of from 3 to 6 feet, 

 rests on abed of kaolin. The basement is very well exposed in some 

 of the railway cutting?, as, for instance, at Baker's Hill (30 miles 

 E.N.E.), noddy's Well (50 miles E.N.E.), Gooseberry Hill (10 miles 

 E.), and other places. The leaching of the rock has been so thorough 



