296 PROP. T. W. EDGEWORTH DAVID ON [May 1896, 



(c) Bacchus Marsh. 



This district was examined by the author in company with 

 Messrs. G. Sweet and Charles C. Brittlebank during December 1894, 

 and again by the author in company with Messrs. C. C. Brittle- 

 bank, T. Brittlebank, Graham Officer, and L. Balfour, during 

 December 1895. The formations represented were as follows : — 



1. Lower Silurian (Ordovician), black slates and shales, with 

 graptolites and grey quartzites intruded in places by granite. 

 The strike is N. 10° E. to N.E., and they dip at a high 

 angle. "Wherever their surface has been freshly exposed in 

 this neighbourhood it is seen to be strongly grooved and 

 polished, and more or less moutonnee. Such striated pave- 

 ments have been traced by Messrs. Sweet and Brittlebank at 

 intervals over an area of 130 square miles in this district ; 

 and later observations by Messrs. Officer and Balfour in the 

 Coimadai district have proved the area to be much greater. 

 In places where the Ordovician clay-slates have had their 

 glaciated surface destroyed by denudation, an exquisite cast 

 of it is preserved on the under-surface of the glacial beds, 

 accurate impressions being retained of even the most minute 

 striae. The surface of the pavement is very uneven, being 

 traversed by troughs from 500 to 600 feet deep, the slopes 

 of the ridges separating the troughs one from another being 

 sometimes as steep as 70°. The bottoms of the troughs and 

 the slopes and summits of the ridges are all strongly glaciated. 

 The author agrees with Messrs. Sweet, Brittlebank, Graham 

 Officer, and L. Balfour in their inference that the ice which 

 produced the glaciation came from the south. At the Wer- 

 ribee Gorge the striae trend from about S. 12° W. to N". 12° E. 



2. Permo-Carboniferous Glacial Beds. — The thickness of these 

 rocks has been approximately estimated, as already stated,, 

 by Messrs. Sweet and Brittlebank to be about 5000 feet, and 

 on the occasion of the author's first examination of them it 

 appeared that this estimate of them was not excessive. 

 Measurements, however, taken by Messrs. Chas. C. Brittle- 

 bank, T. Brittlebank, Graham Officer, L. Balfour, and the 

 author last December show that the thickness may perhaps 

 have been over-estimated through a repetition of the beds 

 resulting from faulting or folding. Their general dip is 

 rather steep, varying from about 15° up to 60°. They con- 

 sist of : — (i) Hard and soft mudstones, from brownish-grey 

 to light claret in colour, bluish-grey at a depth. A small 

 proportion of fragments of undecomposed felspar is present, 

 together with minute chips of black shale (Lower Silurian ?) 

 and small pieces of carbonized plants. The soft mudstones 

 are chiefly composed of clayey material, with quartz-grains r 

 mostly subangular, and contain glaciated erratics sparingly. 

 The hard mudstones contain very numerous strongly- 

 glaciated boulders, frequently flattened on one side as though 



