The Geology and Physiography of the Lawnswood Area. 
61 
Origin of the hornblende schists . — Types (i) and (ii) are thought 
to be the result of extreme regional luetamorphism of basic igneous flows 
or sills, as, according to Wiseman (db, p.394), hornblende Avith a refrac- 
tive index of ^ = 1.679 is indicative of an epidiorite formed in the 
sillimanite zone of metamorphism. Prider (23, ]>. 107) concluded, from 
the high refractive index of the hornblende, the occurrence of tlie rocks 
in beds intei'calated with the metasediments and their chemical compo- 
sition, that similar hornblende schists in the Toodyay Area arc highly 
metamorphosed basic igneous rocks. 
Previously when dealing Avith the correlation of the Toodyay and 
Lawnswood Areas it was painted out that Horizon 6 (see Table 1.) is a horn- 
blende schist in {lie LaAvnSAvood Area and a sillimanite schist in the Toodyay 
Area. The presence of hornblende schist in the same horizon as sillimanite 
schist suggests that this hornblende schist Avas a basic (dolcrite) sill that 
Avas injected in some places into a softer argillaceous stratum between tAAm 
arenaceous bands prior to regional folding. 
The quartz-rich hornblende schist (iii) is thought to be the result of 
extreme metamorphism of a basic sediment (greyAvacke) or basic tuff. 
(b) Cordierite-anthopligllite rock. 
About 50 chains west of Clackline an outcrop of a light greyish-green, 
medium to coarse-grained, uniform-textured cordierite-anthophyllite rock 
occurs. It has been described by Simpson (27, p. 115). It outcrops at the 
contact betAveen the Jimperding Series metasediments, AA’hich are ])eltic in 
this exiiosure, and a rock AA’hich is thought to be the Younger (Iranite al- 
though it is gncissic. 
The minerals are greyish-green cordierite, devoid of cleavage and up 
to three mm. in length, and grey prisms of antho])hyllite up to Iaa^o mm. in 
length. The microstructure is granoblastic gneiasic. Cordierite occurs in 
colourless xenoblasls frequently altered along irregular cracks to pinite, 
and the anthoithyllile is generally idioblastic. ReddiroAvu biotite, rutile, 
and chrnmile are accessory. The approximate composition is cordierite 
(and pinite) 55 per cent., anthophyllite 45 per cent. 
Simpson (27, {i. 116) suggests that this rock originated hy the ^'absorp- 
tion of some slate or similar aluminous rock” into a basic hypersthene-rich 
rock. Similar cordierite-anthophyllite rocks occur as xenoliths in the 
granitic gneiss of the Toodyay Area, and these have been shoAvn to be 
o'pnetically related to an ultrabasic sitinel-olivine-hypei"Sthene rock. Prider 
concluded that a "hypersthenite magma AA'hich had ])een contaminated by 
assimilation of aluminous material” had been altered to cordierite 
antho})hyl1ite rock “by the siinjde addition of silica (probably from the 
granite)’’ (21, p. 381). 
The presence of mica schist adjacent to the coi’dierite-anthophyllite 
rock in the Clackline occurrence suggests that a hypersthenite invaded a 
politic band of the metasediments and assimilated aluminous material, as- 
Simpson suggests. The cordierite was formed either during the highest 
stages of regifuial inetaninrphism or more ]>robably as a result of the contact 
metamorphism (Avith silica addition) by the A^ounger Granite. 
