128 
Rex T. Pkider. 
(6) Ultmbasic intrusioes.-— There are two types of uitrabasi(i iiiti*tisives- 
iu this area : — 
(i) Completely metamorphosed types, now monomineralic tremolite- 
and actinolite schists, occurring as sill-like intrusions into the 
.Jimperding Series. The age of these rocks is unknown, but as will 
be shown below, they are probably earlier than the later normal 
granite. 
(ii) Serpentine dykes. One only of tliese serpentine dykes has been 
noted in the area mapped. It is about -J-chain wide and has the 
same trend as the quartz dolerites. Its age relations with the- 
dolerites are not known. 
(i) Mctmiforphosed idtrabasic sills. -Sevcu'al narrow sill-like intrusions, 
in the (piartzite were noted in the \'icinity of the R. W. Trig station. They 
are jiale greenish, soft, scliistose rocks composed almost entirely of a jmle 
greenish actinolite with Z A c ^ 10'^ ; y ^ 1 • 6o0 ; a = I -030 ; (--) 2V large. 
This actinolite occurs in a felted network of line prisms with the interstict^s- 
filled with a pale green optically positi\e chlorite. Irregular-shaped grains, 
of magnetite are scatter<Hl uniformly throughout. 
Another rock (15(>41), also a sill in ((uartzite, consists largely of a felted 
mass of tremolitt' ])i-isms with a ground of flaky antigorite dusted with mag- 
netite' inclusions. 
A similar type is found just Ixdow the andalusite schist in the S.W . pai't 
of tlie area and this may be the intrusive which has led to tlu' formation of 
andalusite. If so, the retrogressive clianges in the andalusite, induced by 
the intrusion of the normal granite fixes these ultrabasic sills as pre-granite 
in age. 
(ii) iSerjicntiites.- -These rocks are found in a narrow dyke Cchaiu wide 
and the only a\ ailable specimen is from a ]>oiut 2S chains N., 132 chains K., of 
datum. 
It is a fine, even, dark greenish rock with a conchoidal fracture. Numerous 
minute silvery chlorite flakes are visible in hand specimen. Under the micro- 
scope it is seen to be made np of a very tine-grained aggregate of fiak>' anti- 
gorite - 1*57) with occasional relict prisms of tremolite and a later develojx 
ment of chlorite flakes with magnetite inclusions coating the well-marked,, 
crumpled 001 cleavages. 
The chlorite is a very pale greenish variety and has y — 1-595 ; « 
1-589; y - a = -000: and Z 1 001. This mineral, therefore, is a slightly 
aluminous iron-bearing antigorite. 
The rock is a normal serpentiii'/ as shown by the following analysis ot 
Specimen No. 15425. 
SiCb ... 
TiO^ 
AI.O3 
FcO ... 
.\liiO 
CaO ... 
NaoO 
K2O ... 
H,0-b 
B,0- 
MnO 
f.o. 
40-33 
0-28 
2-75 
5-43 
5-48 
33-39 
I -29 
Ad 
Nil 
JO- 16 
0-10 
0- 14 
Ad 
0-10 
Analyst : K. T. Pridcr. 
99-45 
