Physiography of Werrihee Area. 201 



t. Permo-carboniferous sandstones. (Most rupidly eroded.) 



2. Gianodiorite. 



■•5. Ordovician slates and sand stones. 



4. Newer Basalt (Most resistant.) 



(g) Gravels, sands, slays and soils. — This is the latest series of 

 rocks in the area, and their period of deposition extends from the 

 'close of the newer basaltic period up to the present day. They 

 are naturally very loosely compacted, and present slight resistance 

 ^to erosion ; they are found throughout the whole area as small 

 river flats, but are chiefly developed in blocks B, C, and E. We 

 may classify them under three heads aS' regards origin : — 



(i.) River Deposits. — Here we must place all the rivei- flats 

 throughout the area, and particularly the large triangular patch 

 at the mouth of the Werribee River, as well as tliat surrounding 

 ithe You Yangs, 



(ii.) Fault aprons. — These like the last were deposited by 

 :stream action, but are difierentiated in this classification on 

 account of their origin as alluvial aprons, extending outwards h'>m^ 

 the bases of the various dominant scarps. They occur extensively 

 along the west of block E, in the centre and west of block C, and 

 i-n the north of block B, Their exact relationship to the seai'ps will 

 be dealt with later. 



(iii.) As a separate case we may consider the fertile and extensive 

 flats of Bacchus Marsh. They constitute a considerable area of the 

 most productive soils in the State, and in summer, when all the open 

 ■country of the area is dry and brown, the green flats of Bacchus 

 Marsh may generally be detected from any viewpoint in the dis- 

 trict like an emerald set centrally between the Divide and the sea. 



(h) Dykes of various ages. — These occur throughout the area 

 "intersecting the lower Ordovician, permo-carboniferous, and older 

 basalts, but were not seen intruding into any later series, although 

 dykes associated with the newer basalt must necessarily cut through 

 the middle tertiary beds. 



These dyke rocks have not been very closely examined petro- 

 logically, and for our purposes may be i-oughly divided into acid 

 and basic types. The majority of those seen by the wi-iter were 

 east-west, and Avere apparently basic; in most cases they were of 

 lower resisting power than the intruded rock, being marked on 

 the surface by a hollow rather than by a ridge. North-south 

 dykes also occur, a w^ell-known one being in the Wei-ribee Gorge. It 

 is a highly resistant quartz-porphyry, and has noticeably restricted 

 the width of the goi-ge at the point of intersection. 



