Geology and Petrology of Part of Toodyay District, W.A. 85 
This paper has been subdivided into two main sections, thus 
1. The Geological Structure. 
Tl. Petrology — 
(A) The Jimperding Series. 
(B) The later igneous intrusions. 
In view of the variety of rock types developed in the area, theories of 
origin of each main group are discussed immediately after the description 
of typical members of that group and these theories are briefly recapitulated 
at tlie end of the paper. 
II. THE GEOLOGICAL STRUCTURE OF THE AREA. 
The regional strike throughout the Pre-Cambrian rocks in South-Western 
Australia is N.W.-S.E. In the area under consideration, there is a distinct 
departure from this N.W. regional strike, for, over a great part, the strike is 
more nearly E.-W. 
A Pre-Cambrian metasedimentary series (the Chittering Series) has been 
described by Miles (1938) from the Chittering Valley, some 15 miles west from 
the Toodyay area. The dominant strike of these rocks is N. to N.W. These 
rocks are similar in character to the Toodyay rocks, but no information regard- 
ing the relationship of the Chittering and Jimperding Series is available, as 
the country between Toodyay and tlie Chittering Valley has never been 
geologically examined. 
Soutli of tlie Toodyay area, the Jimperding Series has been noted at 
Clackline and York and in both places the dominant strike is in a northerly direc- 
tion. 
It will be seen, therefore, that the Toodyay area is one of abnormal strike 
- -unfortunately, field work has not covered a sufficient area to warrant any 
dthnite conclusions being drawn as to the true character of the structure, 
l)iit in tlie following pages the structure is described so far as it is known. 
From tlie geological map of the area (Plate I. ), it will be seen that the whole 
are'a, except tlie S.W. corner, is occupied by a confoianable series (note, how- 
(‘^■er, that at a position about 300 chains W. and 80 chains N. of datum, the 
out(.‘r band of granitic gneiss appears to transgress the quartzite-hornblende 
s('hist-mica schist bands). The main structural features can be seen best in 
the accompanying sketch map (figure 1), which has been simplified from 
th<‘ more detailed, larger-scale, maj). 
Two wide bands of granite gneiss are interbedded v'ith the metasediments 
-the banding everywhere conforming to that of the metasediments (except 
in the one place noted above). In the south-west part of the area, the bound- 
ai’y between granite gneiss and metasediment on the western side strikes nearly 
E.-W., but farther east the strike swings round to a south-easterly direction. 
The northern Iioundary of this upper gneiss band does not run parallel to the 
southern boundary, except in the westernmost part of the area. Towards 
the centre of the area it swings to the N.E. for a distance of approximately 
thre(^ miles and then turns sharply to a S.S.E. trend. The outer granitic 
gneiss (termed “ Lower Gneiss ” in figure 1) runs more or less parallel to the 
