Geology and Petrology of Part of Toodyay District, W.A. 121 
microscope many of the hornblende prisms carry small rounded poikiloblastic 
inclusions of clear quartz. Rare felspar is evidenced by the occurrence of 
small patches of fine granular epidote. 
Pyroxene-plagioclase amphibolites. 
Tliese rocks are similar to the (piartz-plagioclase amphibolites with the 
addition of a pale greenish diopside. They are like many of the hornblendic 
rocks of the Lewisian of Scotland. 
No. 15445 is typical (figure lOA). Tt is coarse, even-grained, granoblastic, 
Avith no tendency to gneissic structure. The constituents are hornblende 
{(iO j)er cent), diopside (25 per cent) and oligoclase (15 per cent), with 
a'-C(‘SSory tpiartz and apatite. 
The hornblende, in plates to 3 mm. diameter, sometimes moulded arormd 
iiiopside, is a. brownish-green variety similar to that in the quartz-plagioclase 
amphibolite. Tt is noticeably paler in colour at its junction with the diopside. 
Poikiloblastic quartz inclusions are common. 
The pyroxene is a pale greenish diopside with Zac = 38°. It has a 
curious mottled appearance due to small platy inclusions of pale green horn- 
l)lende, which appear to be developing at the expense of the diopside. 
The fels])ar is an oligoclase (Ab^An^) with fine albite twinning, occurring 
in subhedral to anhedral grains which are, in plac'es, associated with horn- 
blende in a sub-ophitic fashion. 
Apatite, in euhedra averaging 0*2 mm, is the most abundant accessory. 
Quartz is very rare. 
Chemical Composition of the Amphibolites. 
ypecimen 15441 (described above) was analysed as representative of 
the most widespread type. The analysis appears in Table 1, column 3. 
The similarity to the schistose plagioclase amphibolites 1 and 2 is at once 
evident. The igneous character of these rocks has been noted on an earlier 
l^age and need not be considered further here. Except for its slightly higher 
alumina and low lime and magnesia, this amphibolite analysis agrees verv 
<'losely with that of the later dolerites (Table 1, col. 4). from this area. 
The am])hibolites, then, appear to be derived from basic igneous rocks 
a])proximating to dolerites or quartz dolerites in composition. Tilley (1921, 
pp. 98-116), Avho has described a number of amphibolite enclosures in the 
granite gneisses of the Southern Eyi-e Peninsula, South Australia, which are, 
in many ways, similar to those found at Toodyay, has outlined (pp. 108-109) 
the chemistry of the conversion of pyroxene to amphibole and little would 
be gaiiK'd by repeating this here. 
Wliere the hornblende becomes more abundant in these xenoliths its 
crj'stals beconn^ larger and apparently more aluminous at the expense of the 
I>lagioclase. Quartz persists, as a by-product of the amphibolitisation, even 
in the almost pure hornblendic types. 
The pyroxene-plagioclase amphibolite xenoliths appear to repres(mt 
completely recrystallised older basic rocks, such as occur in sill-like bodies 
of pre-tectonic age, interbedded with the Jimperding metasediments. The 
grain of these rocks has become coarser as a result of the recrystallisation of 
tlie amjdiibole. Basic xenoliths are often of fine grain (Joplin, 1935, p. 233) 
ascribed in part to the disruption of highly poikilitic crystals of hornblende. 
In the Toodyay gneisses the amphibolites have not suffered any such breaking 
up of the hornblende. 
