12(3 
Rex T. Prider. 
The microscopic characters of c(uartz dolerites have been so frecpiently 
and fidly described that little space will be devoted to the description of the 
Toodyay quartz dolerites which do not differ in any way from the normal 
type of this rock. The only variations are in the proportion of micropeg- 
ma.tite and the degree of deuteric alteration. 
The minerals developed in these rocks are : — ■ 
Pyroxene, which is always a pale brownish, non-pleochroic, monoclinic 
variety, of subhedral to euhedral development, usually penetrated opliitically 
by plagioclase. There is no zoning and only one type is present. Twinning 
on 100 is common. The extinction Z ' c is 38-41° and 2 V is somewhat variable 
in different grains from 42° to 48° (measured on the universal stage) and y 
— 1*720. The data indicate a pyroxene intermediate in character between 
pigeonite and the diopside-hedenbergite series. In some instances, the cleav- 
ages of the pyroxenes are bent and the crystals show incipient fracturing 
(fig, 16A). Alteration to a fibrous greenish uralite is common, proceeding 
from the periphery to the centre and often completely replacing the pyroxene. 
Plagioclase is the most abundant constituent. It is in long columnar 
and lath-like crystals with ^veil-defined edges in the ])rism zone and poorer 
terminal faces. Fine lamellar twinning, both albite and pericline, together 
with simple C’arlsbad twinning are commonly developed. The main felspar 
is a labradorite aljoiit Ab 5 QAu 5 Q, but it almost invariably exhibits a gradational 
normal zoning, which in some instances has been to a periphery as sodic as- 
AbgoAiiQo, although the general range is from AbgQAngQ to AbypAngQ, In many 
instances the plagioclase is completely replaced l3y a fine granular saussurite 
and in such instances there may be a thin outer rim of sodic plagioclase. The 
sassuritisation, although generally irregularly distributed throughout the 
grains, seems to commence at the centre. Arborescent growths of a pale 
greenish weakly birefi-ingent chlorite along cleavages and fractures of the 
plagioclase are very common. The chlorite appears to come from the fibrous 
uralitic coating of the pyroxene and its distribution points to the migration 
of the end stage ^’apou^s and litjuids along the cleavages of the earlier crystal- 
lised felspar. 
Iron ores, mainly ilmenite, may occur in skeletal crystals as large as the 
largest pyroxenes. In some highly uralitised types it is altered to leucoxene 
(pale greyish in reflected light). Magnetite is present in one dyke to such an 
extent as to influence compass readings taken in its immediate vicinity. This 
magnetic feature of the dyke rocks was only noted in this one instance. It is 
probable, howe\’cr. that much of the more common ilmenite has magnetite 
associated with it. The iron ore when in contact with felspar is usually rimmed 
with greenish fibrous chlorite. The only other ore mineral is pyrite, which is 
of rare occurrence, occurring in small cubic crystals. 
Qvariz varies from 0 per cent to 10 per cent. It is in angular grains 
filling the interstices between plagioclase prisms. In the more acidic types 
it commonly occurs in micropegmatite (along with a dusty acid plagioclase). 
Angular grains of micropegmatite up to 2 or 3 mm. diameter have been ob- 
served. It usually carries needle-like inclusions of apatite. 
Amjyhibolc is represented by two varieties : — 
(1) A pale greenish, fibrous uralite with patchy coloration developing 
around the edges of pyroxene plates and frequently completely 
replacing that mineral. Although the edges become frayed, the 
ophitic intergrowth of plagioclase (usually saussuritised in the 
uralitic rocks) with these uralite pseudomorphs still remains visible. 
