THE GEOLOGY OF MITTAGONG. 321 
(2) Berrima Road.—This is a fairly extensive bed extend- 
ing about 100 yards along the road and 50 yards back on 
the northern side. It is not so decomposed as many of the 
tuffs. Fragments of shale and sandstone are enclosed. 
This bed was probably formed at the time of eruption of the 
neighbouring mass of trachyte mentioned previously. 
(3), (4) These tufis are very much decomposed and are 
of little use for stratigraphical purposes. 
1.—d. Trachyte dykes.—The whole district round the 
Gib, but especially for a few miles to the north, is permeated 
with dykes. Over fifty have been mapped by us. As 
before mentioned, these dykes have to a great extent 
determined the contour of the country. Although the 
dyke itself is almost invariably decomposed to a soft trap 
of light colour, yet the intruded rocks have been very much 
hardened, the sandstone into quart zites and shales into chert. 
Hence the hard quartzites resisting erosion have in many 
cases formed the crests of ridges. We found it to be the 
rule that almost every small hill owes its existence to the 
presence of dykes or other volcanic action. 
The areas of subsidence are in many cases bounded by 
long dykes which have evidently intruded the planes of 
weakness. One example of this class occurs runing north 
and south along the slope of the sandstone hill to the west 
of the Gib. Here the junction between the later Wiana- 
matta Shales and much more elevated Hawkesbury Sand- 
stone is marked by a decomposed dyke. The dykes are 
usually three or four feet wide and can sometimes be traced 
for half a mile. 
2.—a. Dolerites.—One of the finest specimens of dolerite 
in the State occurs on part of a small hill now occupied 
by a private garden, about three-quarters of a mile east- 
north-east of Mittagong Station. It is found in form of 
large subangular boulders which are extremely hard. The 
