322 T. G. TAYLOR AND D. MAWSON. 
surface is slightly weathered but inside the rock is quite 
undecomposed. It breaks with a ring like phonolite, and 
pieces fly away to great distances. The dolerite appears 
to have intruded an isolated patch of syenite (much 
decomposed). The actual line of junction is completely 
hidden by the clays etc., resulting from the latter. The 
rock makes a beautiful micro slide, a description of which 
will be found in the petrological section (No. 5). 
In the Nattai River at the foot of the incline leading to 
the Mittagong Coal Co’s. adit, is a large mass of dolerite 
which probably represents a laccolite. Numerous sills have 
spread out horizontally between the Permo-Carboniferous 
Shales. The latter are converted into a black chert some- 
what resembling rhyolite macroscopically. The bed of the 
river from the adit downwards appears to be cut out of this 
dolerite. The latter varies in texture and passes into a 
basalt zone about 20 feet thick at its junction with the 
coal measures, due to its rapid cooling. Several bluffs, 
some 40 feet high, some with prismatic structure, are cut 
through by the river a little lower down. On the east 
bank, 200 yards down from the adit, there are some broad 
dykes of lighter colour, consisting mainly of felspar and 
magnetite. 
‘“‘Qurrockbilly,” three miles along Joadja Tram Line.— 
This prominent hill consists of intermediate rock on the 
east, which is much decomposed, but the lower western 
portion is formed of dolerite and decomposed tufis. A large 
broad dyke appearing as red soil runs about one mile east 
from this centre and a good section (already described) is 
shown in cutting on Mittagong Coal Co’s line. Sills of 
dolerite, two feet thick, in the shaley bed (Wianamatta) of 
Kells Creek, south of Currockbilly, probably originated in 
the above boss. Much of the low lying country in this 
