Geology and Petrology of Part of Toodyay District, W.A. 127 
(2) A brownish well-crystallised hornblende, only found in those doler- 
ites which have abundant end stage mioropegmatite. Although 
at times it appears to be of primary crystallisation, it often 
develops from the outer edge of the fibrous uralite, where this 
is in contact with the micropegmatitic mesostasis. It has pleocliro- 
ism — X yellowish-brown ; Y brownish-gi-een ; and Z bluish-green. 
Biotite is a rare constituent only found in the most acid types, closelv 
associated with the acid mesostasis. 
Olivine is extremely rare, having been recognised in one rock only, where 
it is completely replaced by a dark greenish serpentine. 
Apatite is the only accessory generally developed. It is in needles except 
in the most acid type, in which it builds stout euhedral prisms (fig. 16B). 
Epidote, in addition to its occurrence in saussuritised plagioclase, often 
occurs in narrow veinlets cutting directly across all the above-named minerals. 
Chemical compositioii of the quartz dolerites. 
A typical example (No. 15424) showing only a very small amount of 
quartz and slight uralitisation was analysed and the result appears in Table 5, 
where it is compared witli the average analysis of the Whin Sill rocks. The 
analysis of the Toodyay rock closely approaclies the Whin Sill t;^’pe, differing 
only in the somewhat liigher lime and magnesia in the former. 
TABLE 5. 
Analysis of quartz dolerite from Toodyay compared ivith the average composition of the Whin SUL 
1. 2. 
SiOa 49 
TiOg 
AlgO, 
Fe^O, 
FeO 
MnO 
MgO 
CaO 
NagO 
K/) 
H,0+ 
HgO- 
P.O., 
FeS, 
Others 
1 
13 
3 
8 
0 
7-64 
11-84 
13 
27 
13 
65 
95 
15 
72 
16 
72 
04 
0-14 
0-45 
99 • 99 
50-52 
2-39 
76 
13 
3 
8 
0 
5 
9 
87 
•50 
16 
42 
09 
2-42 
0- 96 
1- 51 
0-76 
0-26 
0-69 
100-31 
Norm : 
Q ... 
Or 
Ab 
An 
(li ... 
hy 
ms 
ir 
ap 
py 
(hiartz dolerite (s[)ee. 15424), Toodyay 
Average ^\'hin Sill Ty])e (Holmes and H 
3-48 
1 -ll 
14- 15 
27-52 
24-88 
18-33 
5-34 
2-43 
0-.34 
0-45 
W.A. Analyst, K. T. Prider. 
arwood. 1929, p. 539). 
The resemblance of the Toodyay tpiartz dolerite to other doleritic rocks 
of the Darling Range near Perth which are considered to be comagmatic, has 
been noted in a in-evious ])aper (Prider, 1941, p. 46). 
