COLONIAL GEOLOGY — MELBOURNE. 



33 



with a corresponding submersion. At the immediate junction of the 

 lately-described Upper and Lower Tertiary beds, for example, not only is 

 the upper surface of the latter waterworu, but the rock itself, for many 

 feet in depth, is fretted into numerous fissures, subsequently filled up with 

 material exactly corresponding with that comprising the rock above ; thus 

 a distinct proof is afforded of the underlying rock having formed an old 

 coast-line, or at least a rocky eminence nearly awash with the waves, from 

 which position the whole afterwards sank sufficiently deep below the 

 water to allow of nearly 20 feet of newer strata being deposited at a later 

 period. 



Still further proof of these changes of level is afforded by certain for- 

 mations around Greelong, at which place thick beds of limestone, contain- 

 ing Planorbis, Lymnea, Paludina, and other limestone and fluviatile shells, 

 appear above Miocene and beneath nearly 20 feet of Marine Pliocene de- 

 posits ; thus necessitating an uplieaval over a portion of the country 

 large enough to allow of freshwater lakes being formed, and a subsequent 

 submersion of the whole district, in order that the marine strata might be 

 thrown down. 



Th6se continued elevations and depressions of surface are not a little 

 puzzling to the tyro scarce willing to admit the possibility of any such 

 phenomenon in a single instance. The district, however evidently vol- 

 canic, would appear to have been especially subject to the changes spoken 

 of, — a rise generally preceding, and a submersion succeeding, each out- 

 burst of lava. Nor is it at all improbable, from recent observations, but 

 that the land over the whole of Port Phillip Bay is still rising,* whether 

 to be followed by a renewed outpouring of igneous rocks and a fresh sub- 

 mersion is beyond the power of science to foretell. 



No account of the Melbourne district would be complete without some 

 mention of the Basalt — the bete noire of the miner, the material of half 

 our public buildings, the substratum of so many Australian plains, the 

 core of so many of our hills, and the allowed source of the colony's fer- 

 tility. This particular rock is of various ages ; no less than three different 

 intrusions or outflows are traceable on the map annexed. The newer or 

 Basalt proper about Victoria, in vast irregularly-shaped patches. On care- 

 ful exploration, these generally show traces of having been ejected in some 

 higher portion of the land, filling up the adjacent valleys with a once 

 molten but now solidified mass. At first view, few mineral substances 

 would seem less capable of being acted upon by the elements. Yet the 

 fertile soil, composed wholly of basaltic clay and sand, together with huge 

 boulders, masses left by the decomposition of the rock around them, 

 show that so indurate a material is by no means proof against such com- 

 paratively weak forces as atmospheric actions. The denuding power of 

 water is also strikingly shown where the Merri Creek has cut its way in 

 a channel, 60 feet deep, through basaltic beds for many miles. Still more 

 remarkable is the denudation of the Keilor Valley. At this place, not 

 only has the Basalt been removed, but 20 feet of Tertiary, and from 30 to 

 40 feet of Silurian rocks have been cut through also ; thus leaving a 

 number of semi-detached hills, each having a base composed of shales and 

 Palaeozoic sandstones, strangely distorted by ancient convulsive move- 

 ments ; a middle portion, formed of Tertiary deposits, lyingun conform- 

 ably upon the older rocks below, and on their summits a thick capping of 



* The ' Lightning,' Black Ball liner, lately struck upon a rock near the Heads, 

 which had never been laid down in any chart, probably because the depth at which it 

 lay had decreased since making the first survey of the channel. 



VOL. VII. F 



