Mineralogy of the Donnybrook Sandstones 
W’^ESTE RN Au STB A L T A . 
217 
stonc-s \v(’iv boini'' laid down. The scju'city oi‘ ainpliibolc, wliit-h makes up a 
very minor part of the r(‘sidu(*s, sug-^'('sts deposition some distance from lh(‘ 
source and also jtossible re-workino* of a se<limentary series. 
Tile above interpretations of the mineralogy must be reg-arded as ten- 
tative only, for the spet-imens examined come tb'oni a rath('r small area. It 
is known, too, that most have come from near the top of the formation, so 
that a xeiy ditfei-ent pictui'e nug’ht be obtained if the lower members could 
also be examined. Nevertheless, this investigation will, it is hoped, provide a 
basis for future work on these sediments, and it is nlso ho])ed that the Collie 
Coal Measures may be examined. It has been suggested, on the om*, hand, that 
the Coal Measures have been ])rotected in a graben (Woohiough, 1916, p. vi), 
and, on the other that they wei’e de[>osited in a separate basin, ("Woodward, 
1894, p. 548). Examination ol‘ the heavy minerals may be expected to indi- 
cate. Avhi(']i of these th(H)ries is correct. If the g-raben theory is supportc'd, 
then the lieavy minei'als may alfoi'd a nu'ans of correlation between the 
Donnybrook sandstones and the Collie Coal Measures. If the scpai-ate basin 
theory seems more probable, then it is unlikely tliat the delrital heavy 
minerals will be helpful in the correlation of the coal measures with any 
l)art of the Donnybrook sandstones. Saint-Smith (1912 p. 22) has suggested 
that the I)onny})rook sandstones are st ratigrai)hically above the. Coal Measures; 
the examination of the detritals in the Donnybrook sandstones suggests the 
possdiility that part of the ( oal i\Ieasur{' sandstones was eroded to form tlie 
Donnybrook sandstones. I’ew (Hd crops, Iat<'ritic cappings, and heavy timber 
make held work dirhcnlt. Also, over a considerable part of the area when* 
Coal ^Measures or Donnyl)rook sandsttuie occur, more rec(*id lake beds fuitlu'r 
obscure, slratigraphical details (Woolnougli, 1916, 9. v.) 
Tlie tact that tlie ]at(*st dissection ot tin* plateau has uncovered and 
eroded large hills in the Pre-( kimbilan along the Preston Valley, and that 
these hills are higher than most of the Donnybrook sandstone outcrops, 
suggests that the lake or estuary in whi('Ii they wer<‘ laid down had an irregular 
floor caused l)y pre\'ious erosion, and tliat sonu* of these older surfaces are 
now being once again eroded, possibly because of a general uj)lift of this 
part of tiio country. Tlie Pre-Cambi'iau rocks on the noilhern side of the 
Preston Valley may have l)een a land surface which effectiv(*ly blocke<l anv 
communication lietwecn the ('ollie ‘‘lake’’ and the Donnybrook “lake.'’ 
Erosion apiiears to have removed a considerable tbickness of the Donny- 
brook foimation, and this has (-ontiihuted to the T(*rtiai'y beds in the coastal 
plain. 
ACKNOWr.EDCMENTS. 
ibis investigation is part of the research woi’k in sedimentary ]>etrology 
initiated by the Commonwealth Pesearclt Giauit to tlie University of V>sterii 
Australia, and is hei'elyv gratefully acknowh'dged. 
M} thanks are due to Professor E. de Ck Clarke for reading through this 
paper and for many helpful suggevstions. 
KEEEKEN'CIES. 
Forman, F. G., 1936: Inspection of Gold Prospecting at Donnybrook: Ann. Frog. 
Report Oeol. Snrrei/ TV. Aust. for 1935, p. 4. 
Maitland, A. Gihb^ 1939: The Donnyljrook Sandstone Formation and its 
Associates: Jonr. Royal Society TV. Aust., Vol. XXV., p. 177. 
