200 Charles Fenner-: 



common with other tertiary deposits of Victoria, there is no con- 

 sensus of opinion as to their age; Mr. F. Chapman classes them as 

 Janjukian (ref. 12, p. 299). They occur mainly around Bacchus 

 Marsh and in the Parwan valley, where they have been preserved 

 by faulting, and at Maude. At Bacchus Marsh they are probably 

 Huviatile, and consist of gravels, sands, cU\ys, lignites, and iron- 

 stones. 



They include valuable deposits of sands and clays (Dog Trap 

 •Gully), limestone (Coimadai), and fire clays (Darley). Their re- 

 sistance to erosion is feeble, and the Parwan valley, as will be 

 shown is an example of a small creek carving out a gi'eat valley in 

 this material in a comparatively shoi-t time. Where the ironstones 

 outcrop, as in cuttings on the Bacchus Marsh-Myrniong road, they 

 appear to be somewhat more resistant, but the iron is rapidly 

 leached out, and tlie clayey residue is soon carried away. Valleys 

 in these rocks are marked by the frequent occurrence of land slips. 

 Wilkinson and Daintree (ref. 56, Note 11) comment on the very 

 easy erosion of these beds in the Parwan valley, and attribute it 

 largely to the high percentage of soluble salts contained in the beds,, 

 chiefly sodium chloride and magnesium sulphate. 



(f ; Newer Basdltic lavO' flows. — This is the most widespread sur- 

 face rock in the area. It covers almost the whole of block E and 

 larg^ parts of B and D, as a great sheet; tongues occur on blocks A 

 and C. As already pointed out, it has largely obliterated the old 

 drainage system and made the reading of the progressive physio- 

 graphy of the area a very difRcult matter. These basalts are usually 

 widely placed in age as " Pliocene to recent." The rock is a dense, 

 well-crystallised basalt, tuffaceous beds occur rarely (there is a good 

 exposure in a Werribee tributary at the foot of Mount Darriwill), 

 and the very scoriaceous material is confined to the cones. It is 

 well jointed, very platy in places (e.g., bed of Werribee below 

 Ballan), and may be classed as very resistant to erosion. 



This estimate of course refers to the rock as it occurs in most of 

 its outcrops, viz., either quite fresh or with negligible decomposi- 

 tion. It forms on the whole a comparatively thin sheet mainly 

 under 150 feet thick with a maxim,um depth of about 400 feet 

 whei'e filling old valleys. 



It is of interest to note here Mr. C. C. Brittlebank's results, pub- 

 lished in the Geological Magazine, 1900 (" Rate of erosion of some 

 Victorian River valleys "), and following on a series of experiments 

 conducted in the Werribee Gorge itself. Mr. Brittlebank places- 

 these rocks in the following order : — 



