Geology and Petrology of Part of 'J'oodyay District, W.A. 103 
Tlie only other constituent is rare, w^elhshaped brow'iiish tourmaline. 
3n one specimen a few stout rods of sillimanite, arranged parallel to the folia- 
tion were noted, but this is the only sillimanite present in my specimens. 
Proceetling in a southerly dirc'ction towards the granite, the andalusites 
become smaller and eventually disappear. Specimens from this part are 
similar to the groundmass of the andalusite schist but have a greater abundance 
of the lino fibrous sericitic bands and knots, lirownisli tourmaline is present, 
l)ut rare. This type of rock persists right up to the granite contact which 
cuts the schists off abruptly to the south. In a series of specimens collected 
throughout this band there was no apparent variation in the tourmaline 
content. The absence of any concentration of this constituent in the schists 
near the contact seems to negative the possibility that it was introduced from 
the granite. At the same time its constancy of character indicates a common 
source for all the tourmaline. Its origin is therefore somewhat doubtful, as 
it may be developed from constituents introduced from the intrusive granite, 
07' it may be original detrital toui-maline wliich has been recrystallised. 
In considering the origin of the andalusite we naturally look first to the 
intrusive granite as the source of heat. The granite is intrusive because : — 
(1) It ti*ansgresses the bedding of the mica schists. 
(2) Several patches of schists up to 20 cliains long x 8 chains wide 
liave been noted wliich are surrounded by granite. 
(3) It shows no foliation like the granitic rocks in the Jimperding 
S('ri(‘s. 
Tliere has been no apparent thermal alteration of the (country I’ocks. 
As shown above, the andalusite^ is abs('nt in the vicinity of the contact and, 
therefore, the genesis of the andalusite could only be attributed to thermal 
alteration by the granite if there were an increase in the pcilitic constituents 
of th(? original st'diment as it becomes more distant from the granite. Althougli 
no chemical analyses are available, microscopical examination shows that 
tlie composition is faiily constant throughout the band, and this rules out the 
granite as the agent which formed the andalusite. 
The granite' on its outer mai’gin has sufferc'd no chilling and is just as 
<*oai'se-grain(‘d as it is at a distances of a (juarter or half-mile from the contact. 
'^Phis suggc'sts that the intrusion of the granite took place at some depth, into 
se'diments which were themselves in a somewhat heated condition. The 
])ossibility of a faulted junction between the granite and schist must, however, 
not l>e ov'erlooked. 
The distribution of the andalusites indicates an accession of heat from 
lu'low. There are sevei-al inti’usive masses, below the andalusite' scliist (figure 
9), which may have su])pli(‘d the lu'at lU'cessary foi' the dc'velopment of andal- 
usite :• • 
(1) Th(* Uppc'r Gi'anitic giu'iss. 
(2) An ultrabasic sill now rejresentc'd by a monomineralic actinolite 
schist. 
(3) Doleritc dykes. Theses could not have* (‘ffectc'd tlu> devoloj)m('nt 
of andalusite as they ai’c post granite in age. wlic'i’c'as the develop- 
ment of andalusite was pre-gi’anite. 
Latc'r retrogi'c'ssive clianges in the andalusite (i.c., replacc'ment by musco- 
vite and sericit(') are ])robably due to pneumatolytic altc'i’ation by va])oui's 
from th(‘ intrusive granite to the south, in a similar manne]’ to the alteration 
of sillimanite'- to muscovite noted by Simpson at Clackline (Simpson, 193(). 
