[Proc. Roy. Soc. Victoria 34 (N.S.), Pt. I., 1921.] 



AuT. IX. — On an Inclusion of Ordovician Sandstone in 

 the Granite of Big Hill. 



By J. A. DUNN, B.Sc, 

 (Howitt Na-tiir.il History Research Scholar, 1920). 



[Read 14th July, 1920.] 



1 — Foreword. 



Big Hill lies some eight miles S.W. of Bendigo, overlooking 

 the wide expanse of undulating plain extending southwards to 

 Harcourt, Castlemaine and Maldon. Big Hill is one of a series 

 of ranges surrounding the saucer-shaped Harcourt granitic area-, 

 all being in the nature of residuals, owing their existence to 

 :the metamorphism and induration of the Lower Ordovician at 

 the contact of the granitic intrusion. The original sediments 

 near Big Hill have been altered in places to quartzite and mica 

 hornfels, chiastolite often showing in the slates. Well preserved 

 specimens of the altered Ordovician are practically unobtain- 

 able, the rocks having been weathered and leached to a consider- 

 able depth, in some places below 490 feet. '• 



The exact line of contact between granitic intrusion and Ordo- 

 vician is, at Big Hill, rather indeterminate owing to the accumu- 

 lation of hill wash and alluvium on the hill slopes, and at the 

 foot of the hills. However, in places large granitic boulders are 

 found protruding above the surface, and by closely following 

 the beds of the small creeks, the limits of the granitic mass may 

 be very closely delineated. 



2. — Ordovician. 



The Ordovician of Big Hill forms the southern extension of 

 the Bendigo Goldfields, and there are probably three hori- 

 zons of the Lower Ordovician represented here — Lance- 

 field, Bendigo and Castlemaine. The well-formed anticlines 

 and synclines so typical of Bendigo extend south-west, right up 

 to the Harcourt granitic mass at Big Hill, remaining undisturbed 

 both in dip and strike at the contact. In fact, the Big Hill range 

 may be looked upon as the southern limit of the Bendigo Gold- 

 fieldS; for the slopes of the range have been costeened and 



