166 R. ETHERIDGE, T. W. E, DAVID, AND J. W. GRIMSHAW. 
River Dam, and about half a mile north of the point where the — 
estuary of that river enters Botany Bay. For the greater part of 
its course it was little more than a ditch, and was tidal for about 
a mile and a half above the point where it joined Cook’s River. 
Its course throughout is almost. entirely over alluvial deposits, 
derived from the denudation of the low hills of Triassic rocks 
(Wianamatta Shales and Hawkesbury Sandstone) which lie to the 
north-west, north, and north-east. It is the alluvial flats which 
lie on either side of the tidal portion of Shea’s Creek, which con. 
stitute the salt swamps referred to above. The surface of the 
swamp is covered by rank grass and salsolaceous plants with a 
thin belt of swamp oak (Casuarina) along its western margin. 
(2) General Geological Conditions.—A glance at the geological 
sketch map accompanying this paper (Plate 8) shows that these 
alluvials form a somewhat delta-shaped area, about three and a half 
miles long from its apex to its seaward termination, and four miles 
wide measured along the shores of Botany Bay. To the west of 
the present canal area, and at a distance of about half a mile at 
right angles to Shea’s Creek, the alluvials are sharply bounded on 
the south-west by Hawkesbury Sandstone, and farther north-east 
by the Wianamatta Shales. Eastwards their boundary is lost 
~ ander the hills of blown sand in the neighbourhood of the Waterloo 
and Botany swamps and Randwick. The excavations for the 
Shea’s Creek canal prove that these alluvials occupy the site of 
what has once been a large indentation of Botany Bay. 
(3) The Section exposed at Shea’s Creek.—The portion of the 
Shea’s Creek canal excavations, specially examined by us, extends 
from the dam five hundred and fifty feet, measured horizontally, — | 
above Rickety Street, as far as the site of a second dam to the 
north-east, a further distance of 2,150 feet. The bottom of the 
canal is being carried to a uniform depth of ten feet below low 
water, and is one hundred feet wide at the bottom, and two 
hundred feet wide at the top. The mean rise and fall of the» 
tide is about five feet, so that at mean high tide there will be a 
depth of fifteen feet of water in the canal when filled. The 
