THE GEOLOGY OF MITTAGONG. 315 
the strata have been forced up by igneous intrusion and 
the Nattai has cut through the overlying Hawkesbury 
Sandstones.”’ 
The second area is reached by way of the old tramway 
which leaves the Berrima road on the right about four miles 
to the north-east, passing over high trestle bridges and 
through a tunnel excavated in the Hawkesbury series. In 
“many of the cuttings towards the end fine specimens of - 
decomposed dykes can be seen. Hrom the end of the line 
a very steep inclined tramway brought the coal up several 
hundred feet from the adit just above the river. <A very 
fine section of the Newcastle series and overlying triassic 
rocks can be seen in the clifi face above the adit. 
The main point of interest in the third area is the occur- 
rence of an impure graphite representing the coal seam 
highly altered by the intrusion of eruptive rocks. 
Ill. HRUPTIVE ROCKS. 
The district is very rich in eruptive rocks of the follow- 
ing types :— 
1. Alkali-felspar, Intermediate series. 
1. Syenite 
san) 
b. Trachyte. 
ec. Intermediate Tuffs and Breccias. 
d. Trachytic dykes. 
2. Basic rocks including Picrites. 
. Dolerites. 
. Hssexites. 
. Basalt. 
. Basic Tuffs and Breccias. 
. Basic dykes. 
exe Qe“ ome 
f. Ultrabasic series of Jellore. 
1.—a. Syenites.—The most important area of this class 
is of course the huge dome of the Gib. Its summit stands 
