IGNEOUS ROCKS OF TASMANIA. 299 
Somewhat similar rocks are found in the Oyster Cove 
district. 
It is proposed that members of this section be afforded an 
opportunity during their stay of visiting Port Cygnet, and 
inspecting this interesting series of rocks. 
The theralitic magma is also represented in Tasmania. 
In this magma, Al, Na +, K, Ca, Mg, are all high, and SiO 
varies from 27 per cent. to 56 per cent. The rocks are basic 
nepheline ones. Plutonic and dyke-rocks have not been 
met with yet in this State. The discoveries hitherto may 
be tabulated as follows :— 
Trachydolerite, at Circular Head and Table Cape. 
Limburgite, near Burnie. 
Nephelinite, Shannon Tier. 
Melilite basalt, Shannon Tier. 
Melilite basalt, Sandy Bay. 
The bluffs at Circular Head and Table Cape were referred 
by the late Professor Ulrech to nepheline basalt, but the 
determination was afterwards withdrawn in favour of apatite, 
which is present in fair quantity. The felspar is plagio- 
clastic, and olivine is abundant. The latter mineral! serves 
to separate the rock from tephrite. The nepheline is not 
easy of detection, lying concealed as it does in the ground- 
mass. Its determination has been confirmed by staining. 
I have this week received a letter from Professor Rosen- 
busch, who, with his usual kindness, has examined this rock, 
and determined it to be trachydolerite. The habit of the 
rock is basaltic. In these bluffs the ordinary Tertiary 
olivine basalt is also met with, and the geological relations 
of the two have not been examined. 
A dyke of limburgite occurs on the Emu Bay railway 
line, 7 miles from Burnie. This felsparless rock is no doubt 
connected, not with the ordinary olivine basalts of the dis 
trict, but with the nepheline-bearing rock of the bluffs just 
referred to. 
Nephelinite, or nepheline-augite rock, occurs at Hunters 
ton, at the top of the Shannon Tier. This rock here is very 
coarse, abounding in long prisms of augite, which have been 
mistaken for tourmaline. Theremainder of the rock consists 
of nepheline, which has largely decomposed to natrolite. 
This zeolite takes the form of radiated aggregates of snow- 
white aspect. I have not been able to match the rock 
exactly with any European occurrences in respect of texture. 
Ours is much coarser in grain, but the structure is that of 
the nephelinite of the Katzenbiickel. 
