EVIDENCES OF CHANGE OF LEVEL. 533 
that there are seldom accumulations of debris at the base of the cliffs. 
In some places, this action has separated islets from the coast. At 
Wollongong Point, a rude column and hills of rock stand upon the 
platform, the wear having removed the rock within, so that at high 
tide they are cut off from the main land. Such examples are not 
uncommon. 
Other effects of the sea in removing dikes of basalt are of frequent 
occurrence in Illawarra. One of the most remarkable instances occurs 
in a cliff eighty feet high, where a trench eight feet wide, narrowing 
to four, extends into the cliff one hundred to one hundred and fifty 
yards. ‘The waters rush in below, and are still engaged in removing 
the dike. ‘This, though appearing to be an example of valley-making 
by the sea, in fact shows how little is done through this agency : for 
we have here a removal of one hundred yards or more, in a single 
narrow line, scarcely wider than the original dike, and therefore with 
hardly any action on the sides of the narrow channel. ‘The force of 
the waves is expended wholly against the farther extremity of the 
channel, and very little laterally. The action is in the same direction 
as on the open shores. As a line of coast receives the force of the 
waves with less obstruction than a narrow channel or bay, the former 
will always experience a greater amount of wear, except when, as in the 
case before us, there is a channel of different and more yielding mate- 
rial to be removed. ‘The basalt is easily removed by the sea, because 
of its many fractures and its non-adherence to the walls of the dike. 
VIII EVIDENCES OF CHANGE OF LEVEL. 
It is probable, from the absence of more recent deposits between the 
Sydney sandstone and the tertiary, that these beds appeared above 
the waters in Eastern Australia soon after their formation; and the 
great abundance of salt lakes and briny impregnations over the 
country were probably derived from the previous oceans. Since that 
period the denudation spoken of in the preceding chapter has been in 
progress. Subsequent changes of level have taken place, as is appa- 
rent in the coral reefs of the shores north of the parallel of twenty- 
eight degrees, in the tertiary of the southern and western shores, and 
certain terraces and shell deposits along the coast. 
The coral reefs indicate an extensive subsidence along the east and 
northeast coasts of New Holland, (page 399,) the amount of which we 
have had no means of definitely ascertaining. That it must have been 
134 
