GEOLOGY OF THE MELBOURNE DISTRICT. 151 rupture took place, could not be seen, and the usual hydraulic test of twice the working pressure might have strained the part without making the defect manifest. It would further be observed that the shell plate was ripped from the front end plate at the flange seam and straightened out for nearly half the circumference of the boiler. For a thirteen feet boiler shell at 16o lbs. working pressure this plate would be about I4 inch thick. The lesson to be learnt from such a sad occurrence was that careful tests should be made of all material used in such structures, as well as the necessity for vigilant supervision of workmanship during construction. LECTURE. THE GEOLOGY OF THE MELBOURNE DISTRICT. Abstract of Lecture delivered by Mr. T. S. HALL, M.A., D.Sc. The examination of the sections displayed in railway cuttings along our suburban lines affords an easy way of obtaining a view of the geological structure of the Melbourne district. Let us take the Camberwell line. After leaving the Flinders Street Station we have a level run to East Richmond. Bluestone boul- ders near Hoddle Street show us that we are on a bluestone plain. At East Richmond we pass through a cutting in the bedrock of Melbourne, the Silurian, or, as our older geological, maps have it, the Upper Silurian. Then we traverse the blue- stone plain again as far as the Yarra Bridge. The bluestone, or basalt, has flowed down as a lava flood from the northward, and occupies the old Yarra Valley. The bed of this valley is below sea level, as can be seen at the Burn- ley and Clifton Hill quarries, where the old, black, river-silt is deeply buried beneath the once molten rock. The Yarra now flows along the geological boundary, and has worked more towards its left bank, and so formed the steep cliffs that extend from Alphington downwards. On leaving the lava plain at Burnley, and crossing the river, a cutting is necessary to pass through the river cliff, and so strike the valley of the Hawthorn Creek, up which the line then runs. At Auburn we again see the Silurian in the shallow cuttings, and near at hand are brick-pits which are worked in this old bedrock. At Camberwell we enter cuttings in red sands and gravels. These overlie the Silurian as a cap. In a cutting in Royal Park the sands are on decomposed basalt. But at Green Gully,
Transcript
GEOLOGY OF THE MELBOURNE DISTRICT. 151
rupture took place, could not be seen, and the usual hydraulic test of twice the working pressure might have strained the part without making the defect manifest.
It would further be observed that the shell plate was ripped from the front end plate at the flange seam and straightened out for nearly half the circumference of the boiler. For a thirteen feet boiler shell at 16o lbs. working pressure this plate would be about I4 inch thick.
The lesson to be learnt from such a sad occurrence was that careful tests should be made of all material used in such structures, as well as the necessity for vigilant supervision of workmanship during construction.
LECTURE.
THE GEOLOGY OF THE MELBOURNE DISTRICT.
Abstract of Lecture delivered by Mr. T. S. HALL, M.A., D.Sc.
The examination of the sections displayed in railway cuttings along our suburban lines affords an easy way of obtaining a view of the geological structure of the Melbourne district. Let us take the Camberwell line. After leaving the Flinders Street Station we have a level run to East Richmond. Bluestone boul-ders near Hoddle Street show us that we are on a bluestone plain. At East Richmond we pass through a cutting in the bedrock of Melbourne, the Silurian, or, as our older geological, maps have it, the Upper Silurian. Then we traverse the blue- stone plain again as far as the Yarra Bridge.
The bluestone, or basalt, has flowed down as a lava flood from the northward, and occupies the old Yarra Valley. The bed of this valley is below sea level, as can be seen at the Burn-ley and Clifton Hill quarries, where the old, black, river-silt is deeply buried beneath the once molten rock.
The Yarra now flows along the geological boundary, and has worked more towards its left bank, and so formed the steep cliffs that extend from Alphington downwards.
On leaving the lava plain at Burnley, and crossing the river, a cutting is necessary to pass through the river cliff, and so strike the valley of the Hawthorn Creek, up which the line then runs. At Auburn we again see the Silurian in the shallow cuttings, and near at hand are brick-pits which are worked in this old bedrock.
At Camberwell we enter cuttings in red sands and gravels. These overlie the Silurian as a cap. In a cutting in Royal Park the sands are on decomposed basalt. But at Green Gully,
152 VICTORIAN INSTITUTE OF ENGINEERS:
Keilor, we see that the lava of the plains overlies the red sands, and these again another sheet of basalt. So the succession is :— Server Volcanic.
Red Sands (Kalimnan—? Miocene). Older Volcanic (? Eocene). Silurian.
The red sands about the heights of Essendon, Kew, Auburn, Camberwell, Toorak, Malvern, and so to Caulfield and the Brigh-ton-Beaumaris coast form almost level country, with a gentle tilt seawards. It was originally one great sheet, and is now trenched by the Yarra and other valleys, such as that of Gardiner's Creek.
The torrential streams which rose in the Divide to the north, spread their load of sands and gravels over the lower land to the south, and part of it over the sea bed. That laid down on the land forms a "piedmont plain" ; that laid down under the sea, and now in part raised above it, forms a "coastal plain" in the strict sense of the term. The beds of the coastal plain contain marine fossils, but the shifting inland boundary of the two serres has never been even approximately determined.
Thus the essential character of the area east of the Saltwater is a plain, modified by stream action, which has removed the cover over large areas. West of the Saltwater, and forming much of the country to the north, is a lava plain.
The two plains differ in their vegetation and in the contour of their valleys. The impermeability of the basalt to water, and the thin soil cap prevent the growth of trees. Only annual plants can succeed, for the droughts of summer kill off all shrubs. On the western side the Silurian and sands store water in sufficient quantities for trees to grow, and we can still see the character of the native vegetation over much of the area.
The lava flows do not extend up the Yarra beyond Alphington, and here the obstruction to the flow has ponded back the river, and produced the wide flats which reach nearly up to Templestowe, and abound in billabongs.
A third type of plain—a small one—is represented by the delta-deposits of the Yarra-Saltwater system, which extend from Foot- scray to St. Kilda. Over the sea-front sand dunes occur, and these are characterised by a peculiar flora of sand-staying plants.
The terraces of the Moonee Ponds indicate an alteration in the drainage which must have been caused by an elevation of some twenty feet in comparatively recent times.
The lecturer illustrated his subject, point by point, by about 5o lantern slides and sections explanatory of the salient features described .
DISCUSSION.
The PRESIDENT said Mr. Hall had referred to the probable depth of the silt deposited in the old estuary at the West'Melbourne swamp. He knew that, when testing for possible foundations
VISITSM
MELBOURNE TELEPHONE TUNNELS.
On Tuesday, September 29th, members inspected the tunnels which are being driven beneath Melbourne for the accommoda-tion of the telephone and telegraphic services. The invitations from the Commonwealth Public Works Department and from the con-tractor (Mr. A. G. Shaw) were conveyed through Mr. T. Hill (C.P.W.D. designing and supervising engineer), and Major H. V. Champion (consulting and supervising engineer' to the con-tractor), respectively. By the courtesy of Messrs. D. A. and J. W. Swanson, members had also the privilege of accompanying —under the guidance of the Architect, Mr. J. S. Brittingham-the Royal Victorian Institute of Architects through the new tele-phone exchange buildings.
Briefly the salient points of the tunnel system are as follows :-
OBJECT : The ultimate complete undergrounding of the telephone and
telegraph lines of the central—"city"—area of the metropolitan district and connection with suburban arterial systems and with minor conduit reticulations within the city proper. The tunnels are for these purposes exclusively.
VISIT-MELBOURNE TELEPHONE TUNNELS. 153
for railway purposes, some years ago, three 70 feet piles spliced end to end, had failed to bottom at some parts.
Could Mr. Hall enlighten them as to whence the great flow of lava—"bluestone"—which filled the old drainage valleys in the Melbourne district had proceeded?
Further, whether the Yarra, at its present rate of flow, was sufficient to account for the subsequent erosion?
Mr. T. S. HALL said there was considerable evidence that the rivers were formerly less than they were now. But, at the same time, the elevation of the land had been 170 or 200 feet greater, and with the steeper grade the cutting power of the streams must have been greater also. It was the velocity of the water, and the chance it had to discharge its load, rather than the volume, which determined the erosion. He did not think there was any need to call for a larger discharge if they assumed the steeper gradient.
As regarded the lava : It was surprising to note how many old volcanoes there were near Melbourne. Although nearly all very much denuded, there were at least one hundred of them in the county of Bourke, but he could not refer to any particular volcano as the source of any particular lava flow.
