323 
Obsidian. 
Associated with the pebbles of black basalt along the course of Coliban 
River, from the Upper Coliban Reservoir to Taradale, are well rounded 
pebbles of black obsidian. They were first obtained by the writer four 
years ago at Taradale, but they were assumed to be black basalt. 
Recently their true character was recognised, and an analysis was made. 
No pebbles of obsidian were found above the outlet of the Upper 
Coliban Reservoir. The obsidian is a dull black in colour, with conchoidal 
fracturing all over the surface from impact with other pebbles. It breaks 
with perfect conchoidal fracture, is jet black on fracture, and resembles 
bitumen in lustre; it is opaque to semi-opaque. Occasional examp^s of 
red and black obsidian occur. The obsidian pebbles have a peculiarly 
rough feel, while those of black basalt are smooth to the touch. 
There appears to be every gradation from black basalt having uneven 
fracture and dull lustre to> obsidian with conchoidal fracture and vitreous 
lustre. Examples are to be found showing the various stages. The 
obsidian pebbles are more spheroidal in form than those of black basalt. 
The difference in material may account for this. None of the pebbles of 
obsidian were found as much as i lb. in weight. They are far less 
abundant than the pebbles of black basalt, and would not amount to i per 
cent, as compared with them. 
From the association of obsidian and black basalt pebbles along the 
course of the river, and from the manner in which the black basalt grades 
into obsidian, it is to be inferred that both originate at the same locality. 
Obsidian was not observed in situ where the black basalt is found in place, 
but careful search would probably result in its discovery. 
The analysis of the obsidian was made at the Geological Survey 
Laboratory by Mr. J. C. Watson, and the result is given below. For 
comparison, No. 7 analysis of an australite from the table on page 8 of 
Bulletin 27 is placed beside it :— 
(No. 1) 
Taradale Obsidian. 
(No. 2) 
No. 7 Australite. 
Difference between 
1 and 2. 
Range of difference 
in 9 Australites. 
SiO, 
63-67 
per 
cent. 
64'68 per cent. 
1*01 per 
cent. 
13*04 per cent. 
Al-A 
15-83 
3 3 
5 5 
18-80 ,, „ 
0-97 ,, 
3 3 
6 83 „ „ 
Fe 2 0 3 
1-39 
5 3 
3 3 
6-57 „ „ 
5-18 „ 
3 3 
6-30 „ ,, 
FeO 
4’06 
3 3 
3 3 
1-01 ,, 
3-05 ,, 
3 3 
4-29 „ „ 
MgO 
215 
3 3 
3 3 
2-50 „ ,, 
0-35 ,, 
3 3 
1 ’02 ,, ,, 
CaO 
3-88 
3 3 
3 3 
3-8S „ ,, 
0*00 ,, 
3 3 
1-4S „ ,, 
Na^O 
357 
5 3 
3 3 
trace 
3-57 ,, 
3 3 
1*76 „ „ 
KjO 
3-69 
3 3 
33 
4-01 ,, ,, 
0-32 ,, 
3 3 
3-08 ,, ,, 
h 2 o + 
0-02 
3 3 
3 3 
. . . 
h 2 o- 
0 15 
3 3 
3 3 
• . . 
... 
TiO 2 
1-27 
3 3 
3 3 
. . . 
• • • 
... 
B 2 U 5 
0-02 
3 3 
3 3 
... 
• • • 
MnO 
0-43 
3 5 
3 3 
• • • 
NiO&CoO 0-0L 
3 3 
3 3 
... 
... 
... 
10014 
99-45 
... 
... 
Sp. gr 
2*569 
• • • 
From the above analyses it is apparent that No. 7 australite of page 8, 
Bulletin 27, differs less in composition from the Taradale obsidian than it 
does from some of the other australites. Only one example of this obsidian 
has been analyzed, but if a series of analyses were made it is probable that 
