42 T. W. E. DAVID. 
Blue Mountains proper with the elevated land to the north and 
south of them, as far east as the monoclinal fold at Lapstone Hill, 
(2) the plain between Lapstone Hill and the coast, (3) the con- 
tinental shelf. [See Plate 2, diagram 1.] 
(1.) The plateau of the Blue Mountains proper consists of a _ 
deeply eroded platform of Hawkesbury Sandstone. At the top | . 
of the fold at Lapstone Hill the platform attains an altitude of 
about six hundred feet above the sea, and from here it rises 
westwards at the rate of about one hundred and sixty feet to the 
mile. Its greatest elevation on the portion traversed by the 
western railway being 3,658 feet at Clarence Siding. The plateau 
rises in conspicuous peaks at Mount Tomah 3,276 feet, Mount 
Wilson, Mount King George 3,470 feet, and Mount Hay 3,270 
feet. The portion of the plateau which has suffered least from 
erosion, with the exception of the peaks just enumerated, is 
the ridge known as the Darling Causeway, which for a distance 
of about thirty miles forms the line of water-parting between the 
tributaries of Cox’s River and of the Grose River. The course of 
the western railway line and of the main western road, almost 
exactly follow the trend of this causeway. Westwards the 
plateau terminates in sheer precipices of sandstone, from two 
hundred up to nearly 1,000 feet in height, of which Hassan’s 
Walls may be taken asatype. The creeks which drain southerly 
from this water-parting into the Cox, and those which flow 
northerly to join the Grose, within a few miles of their sources 
plunge over sandstone precipices, forming waterfalls of which 
Govett’s Leap, Leura Falls, and Wentworth Falls may be taken 
as types. From the bases of these waterfalls the creeks find their 
way among masses of densely wooded talus into more or less wide 
valleys sloping gently eastwards. Traced a few miles further 
east the rivers formed by the junction of these creeks become 
hemmed in by walls of sandstone, which form almost impassable 
gorges near the spots where the rivers break through the mono 
clinal fold at the eastern margin of the plateau. This remarkable 
contraction of the valleys has been commented on by Darwin in 
